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j —SCREW PROPELLER.—Could one of your oadcr.-i toll nio bow to obtain the pitch of a screw It f—K. W.

,]—BICHROMATE BATTERY.—Will "Sigma" other brotber correspondent kindly tell me what iroper distance to leave between the plates of a 11 ate battery'? The plates are Sin. by 2iu. Also

fbe best way to fasten the copper wire to the

plate ?—Thomas J. O'consob.

.J —HOROLOGICAL.—Would aomo horological ber kindly inform me by what rule watchmakers ict watches—viz., what is the difference between v 1m. 5-10th and one Int. 9-10th? Have they the number of teeth und leaves to wheels and .? Also, how many sizes of watched do the trade Ixo ?—B. E. Loddy.

• 0— BLEACHING IVORY.—Can any subscriber inform mo how the handles of table cutlery that

ecome yellow with use, may be restored to their r whiteness /—black Raven.

^—WATER-WORKS.—Can you or any of your pondents inform me the cost of laying mains in < and roads per mile? In supplying water by gravi, what bead is required to overcomo the pressure pipes ?—T. W. C.

7-3 —TO "INDUCTORICM.*'— Will "Inducto

* state the size of secondary wire he uses? and as I . allows Jin. between the discs, must it be wound rards and forwards in this space until filled, and ,lie next, and so on ?—T. W. C.

t8-]—SPECTROSCOPE.—Can any of your oorre.ents give me instructions and drawings for making jtroscope?—T. W. C.

»y.]—SCIENCE AND ART EXAMINATIONS.— [ ask Mr. G. E. Davis what books he would recom. to one preparing fur the Organic Honours Paper, it May's examinations ?—Caow Tubes.

DO.]—COPIES OF COINS IN ELECTROTYPE. LI some kind brother please tell me how to take is of coins, &c, in electrotype?—William S.

.'CHBtt.

01.]—CLEANING COINS.—Will you or one of your \ readers please to inform me how to clean old 3 (silver and copper) without Injuring the coins?— LI Am S. Fletcheu.

Wa.]— TRUE HEART FOR TRAVERSING.—I Id be grateful if aay one would inform me, by a

1 sketch or olherwfse, how to describe a true heart traversing purposes, from lin. to Sin.—Good

FOB.]—CHANGE WHEELS.—My thanks to "B. H." As auswer to query 4H71, p. 454, but he evidently iikos what I want. He gives me an auswer to lge from one hank to another. I want to know how find- the proper wheels in the first instance. I « never seen a rule myself, and don't know whethor e is one or not. If "B. H." could assist me any her I should bo obliged.—Good Wobds.

704.]-CHANGING SURNAME.—I wish to change surname. How cun I do it?—M. Spieobi.ilai.ter.

705.]—NATURALIZATION.—What step3 must I to become a naturalized Englishman?—M. Spiegel.

700.]—PHOTOGRAPHY.—I put the water that I b the silver prints in into a deep mug and precipitate silver with common salt, and have done so with the itest success for two years. The silver precipitated Hlfully, but now it is just the reverse, it won't pertato at all, and I have put lots of salt in, but to no 1. Would some kind reader of the Mechanic be good ugh to tell me the reason, and if there is a cure, what ; ?—In A Fix.

707-]—STEAM.—Can any of our numerous readers me what would be the number of cubic feet of steam minute, issuing from a pipe 2£in. diameter, under a 4sure of 401l>, per square inch? ,Would they also give rule and wurkiug of the Bame, as I have no books iting on the subject? And also say where I could a work on the subject ?—E. J. Jackson.

4708.]—LOSS OF VOICE.—For the last two or three rs my voice has boen very husky and indistinct, and Hive generally to give a short cough before lean uk a word. I sh;ill i.e very glad if somo kind reader 1 toll me what will improve it.—A Fobesteb.

4709.]—TO MR. PROCTOR.—Will Mr. Proctor kindly uisli me with the dimensions of the stand figured in "Half-hour« with the Telescope," p. 17, and say itUer he thinks it would be suitable for an instrunt 4Jn. aperture 5ft. long? I wish to make a stand ch ?hall giw a steady motion, and be easily moved n. place to placo. Porhaps Mr. Proctor's experience y enable him to suggest some simpler plan that would moderately efficacious.—Turton.

4710]—FASTENING BRASS TAPS IN KITCHEN ILER.—Chd any of your numerous readers inform how to tighten a brass tap to kitchen boiler that has

;oine loose ?—Isqdireb.

4711]-COLZA OIL.—Will any brother reader bo id enough u» imtruet me how to test the quality of Lxa oil—M. Q, W.

;4712.]-ATRv. STEAM.-I want to substitute air steam (like many others I suppose) but there is supposed impossibility to contend with—namely, it it require* just m much power to force the air into i boiler as it does to move the engine. Now if s difficulty could be overcome, could any corresponit tell me, if air even then could cope with steam ?— ices.

l^^'T0^1^.—I TMh to remove my hive inside a .lding during t he winter. Can any reader inform me lie bees would pass in and out through a small hole -de ui aOm. bri«;k wall, and how bees are fed, or what nould feed them with in the winter? I think a few •ersonbees, through the Mechanic would be found ry acceptable to many of its readers.—J. G, 4TM.] -HOT 0 R COLD BATH.—Can any reader give information cm the following points? 1st. What aus could I adopt in order to get hot water from a

wash-house copper (on the ground floor) to a bath (on the first-floor), without carrying it? Will a pump auswer the purpose? If so, what sort? I have an idea that hot liquids caunot be pumped, as they would injure the pumps; is this idea correct, and if so what other expedient can I adopt? If, however, hot water can bo pumped, I presume that the same pump will also serve to pump up cold water from the butt? 2nd. Would india-rubber or gutta-percha tubing answer? Any information on these points would obligo ?—A Handy Man.

[4715.]—SILK DRESSING MACHINE.—Having seen a letter in the Mechanic for June 10th signed E. Davis, I should bo much obliged if he would, through the Mechanic, furnish mo with the particulars regarding a silk dressing machine he mentions, on which he can change the sheets of silk in a few minutes.—A. T. T.

[4716.]—FINISHING BLACK CLOTH.—Can any of

your subscribers inform me of the best method of putting a bright face or finish upon black cloth?—J. R.

[4717.]—BINDING MAGNETIC WIRE.—I believe it is essential that the winding of the wire for magnetic purposes should be performed as evenly as possible. Like a reel of cotton, I find it difficult, or, indeed, impossible, to accomplish it merely with the hand, as it is produced by the regular manufacturer. Would some one kindly inform me what means to adopt to effect my object ?—Magnet.

[4718.]—VARNISH FOR BRIGHT STEEL WORK. Can you inform mo of any preparation for covering bright steel work, so as to prevent its rusting, and at the same time preserve its bright appearance; something that will have the same effect as varnish on paint, enabling me to wash the work when done ?—Thob. Pbiou.

[4719.]—ELECTRO PLATING.—Will some brother reader kindly inform me how I can electro-plate Bmall articles so that they may keep their colour? The articles are brass and copper.—Scbawleb.

[4720.]—KALEIDOSCOPE.—Will any of our optical friends tell me how to make a kaleidoscope such as I see in the shop windows; I don't mean the common ones?-H. K. L.

[4721.]— FOSSILS.—Will Mr. Underfaill tell me whether I can obtain many fossils In the neighbourhood of Hunstanton and along the coast to Cromer? What are they; and where ought I to look ?—H. TJ.

[4722.]—CONSTRUCTION OF SHIPS.—I should be obliged if any correspondent will tell me what is ScottRussell's "wave-line principle" for the construction of vessels.—N. A.

[4723.]— WATER BAROMETER—Will "Cornubia" (p. 390), kindly answer me the following questions :— 1st, What a thermometer has to do with graduating the degrees on hut barometer? 2nd. If the heat would not have any effect in expanding the air contained in the glass bulb, and so effect the height of the column of liquid in the glass tube? 3rd. Why not graduate the degrees on the tube at once, instead of having the revolving apparatus ?—Gsomethos.

[4724.]—CONSUMPTION OF COALS.—Some time ago there was a commission appointed to inquire into the consumption of coal. Can some one inform me if that commission has yet published its report? And If it has, where, and at what price it may be obtained? Or what amouutof coal is consumed in the United Kingdom, in the working of steam engines of all classes, stationary, locomotive, and marine ?—Petruchio.

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Cotton Spinning, p 406.

Fixing and Colouring Prints on Glass, 406.

Water Gilding, 406.

Associate of Art Degree, 406.

Alabaster Glass. To Mr. J. Leicester, 406.

Alloys for Tin Foil, 408.

Cotton Spinning, 406.

Force Pump for Irrigation, 406.

Friction in Steam Cylinders, 406.

Millers, 406.

Brewing, 406.

Silvering Clock Dials, 407

Sausages, t£c, 407.

Madagascar Matting, 407.

Cast-Iron, 407.

Model Mills, p. 430.

Destructive Soap Powders, 43J.

Emigration, 430.

Rocking Chair, 430.

Cleveland Ironstone, 430.

Sizing, 430.

Poluriscope for Lantern, 43U.

Want of. Pressure, 430.

Stone for Trinkets. 431.

Black Glass or Antimony, 430.

Lancashire Black, 430.

Hollow Candles, 430.

Draught Furnace for Smelting Lead Ore.

La Crosse, 430.

Windmill lor Working Lathe, 431.

Cracked Boiler Plates, 431.

Concertina, 431.

Heating House with Hot Air, 431,

Barometer, &c, 431.

Case (or F*rns, 431.

Weaving, fel.

DOMESTIC RECIPES.

From the Food Journal.

VEGETABLE SOUP.—Place one onion, cut in pieces, in a saucepan, with 3oz. of butter, and brown it; then add finely-shredded turnips, parsnips, leeks, carrots (also a white carrot, if procurable», green celery, a little sorrel, one small potato, and a few French beans and green peas, all of which are 'to be tossed and browned in the butter, with sufficient quantities of salt and pepper. Pour in a little water or stock, crush the vegetables slightly, then fill up with inoro water, and let the soup simmer for two hours. A little rice or poarl barley can, with advantage, be added to tho vegetable. Care must be takon not to get the soup of too thick a consistency.

TO KEEP MUSHROOMS WITHOUT PICKLE.—Let the mushrooms be peeled, and the iasides taken out; then lay them to soak in water for two or three hours, after which they muBt be dried. Lay them on tin plates, and set them up in a cool oven, an operation which must be repeated several times, until they are thoroughly dry. They must then be put in pots, and kept from damp.

TO MAKE KETCHUP.—To a peck of mushrooms, put two handfuls of salt. Let them stand two days; then strain them out, and boil for half or quarter of an hour. Scum them well, and season as you like with ginger, pepper, cloves, and garlic, all of which must be boiled but the garlic; let them stand for a day or two, and then put into bottles with sweet oil, tying over some perforated paper.

A COUNTRY CAPTAIN.—Cut a fowl to pieces as for a fricasee; salt, pepper, and cayenne pepper it to taste, The fowl must be fried in butter by itself; three or four large onions fried brown in butter are to be sent up with it, with fried crumbs of bread, lemon peel, and gravy as for a roast fowl.—From an old MS.

A TURKISH BROIL.—Cut from a small cod-fish as many slices as are required, rub them with a little salt, and dip them in flour; then rub a gridiron with olive oil, lay the fish over, set it on a moderate charcoal fire, and turn the fish often. When nicely browned on both sides, take it off, then cover the surface of a dish with some finely chopped parsley and onions, place the fish over, with one or two lemons cut in quarters round the dish, and serve hot.

USEFUL AND S0LENTLTI0 NOTES.

RUSTIC PICTURE FRAMES.—Rustic wood for this and other purposes is in great favour nowadays. According to the Scientific American^ with a little care in selection of material and skill in handling tools, we may frame our engravings and paintings at slight cost. Oak wood, denuded of the bark, presents a beautifully corrugated surface, out of which the knife easily removes the few fibres which adhere, and it is ready for varnishing as soon as it is seasoned. The " season cracks," should they occur, may be filled with dark brown putty, and will even heighten the general effect. Take a thin board of the right size and shape, for the foundation or "mat; " saw out the inner oval or rectangular form to suit the picture. Nation the edge a rustic frame made of the branches of hard, seasoned wood, and garnish the corners with some pretty device, such, for instance, as a cluster of acorns. Ivy may be trained to grow around these frames with beautiful effect.

PLATING COPPER AND BRASS WITH ZINC— According to Buttger, copper and brass can be easily coated with zinc by immersing them in a boiling bath of sal ammoniac containing zinc foil or powder. The deposit of zinc made In this way is brilliant, and adheres firmly to the copper and brass. "Whether iron could be coated or galvanized in the same way is not stated by the author, though the use of sal ammoniac in the ordinary process is well understood. He first prepares finely-divided zinc, by pouring the molten metal in a previously strongly-heated iron mortar, and stirring until nearly cold. The pulverulent zinc thus obtained is placed in a porcelain vessel, and to it is added a concentrated aqueous solution of sal-ammoniac. This mixture is heated to boiling; and the copper or brass objects to be coated with zinc (but previously wellcleansed, and, best, even with an acid) are then placed in tho liquid, wherein they become coated with a brilliantly-white adhering layer of zinc.

METHOD OF PURIFYING ILLUMINATING GAS. E. Pelouze modifies the Lamming mass now generally employed for the purpose of purifying illuminating gas, by adding sulphuric acid. He sprinkles the purifiers containing oxide of iron and sawdust, with water, to which 20 per cent, of sulphuric acid, of specific gravity 1*53 (58J Bauincj, has been added, aud after the mass has dried up sufficiently, passes the gas through. After use it is necessary to restore the sulphuric acid, and to remove the sulphate of ammonia that may have been formed. It is said that napthalin is not removed from the gas by this method. A mixture of oxide of iron und sawdust thus prepared would serve au excellent purpose as a disinfecting agent to be added to the earth in the earth-closet, in stables, cesspools, and the like, but where it has to coine in contact with metals care must be takon not to have free sulphuric acid present. Ordinary copperas or sulphate of iron and sawdust, with native hematite, or bog-iron ore, would al*o make a valuable disinfecting mixture, and would be nearly as cheap as any of the others.

NEGLECT OF MUSHROOMS.—How is it that mushrooms should bo used so timidly, and left to rot by thousands in field aud forest, when, with a rise in the price of flour, or of meat, they might and ought to givo dinners for numberless cottagers, whose children could gather these " vegetable beef-steaks" with ease and with profit? Is there not tho common mushroom, and the l'airy-ring mushroom or Scotch bonnet, the blcwitt and the orange-milk mushroom, the morel—and might there not bo tbo truffle, bad we less pride in borrowing from our neighbours, and more ingenuity in utilizing tbe loan? How U it that Franco is ahead of us in prizing these fungi as delicacies, and Italy very far ahead, so that £4000 worth yearly are sold in tbe market-place of Rome? Wo, says the Food Journal, pride ourselves on our roast beef; let us not have to blush for the scantiness and dearness of our vegetable accompaniments. M. Roques ha*called mushrooms, "the manna of the poor," and yet in England the poor do not consider themselves divinely chosen enough to gather this

CARDED OAKUM A8 A SURGICAL DRESSING.— Se many advantages are possessed by carded oakum as a surgical dressing that we (Lancet) expect it to be largely employed iu the military hospitals of France and Germany during the present war. As, however, its qualities are not yet so widely known as they should be, it may be useful at the present juncture to refer to them. The material is simply old rope shredded in prisons and workhouses, and carded by machinery. It is of a bright brown colour, with the well-known tarry fragrance. A little of the oakum is roughly drawn into a suitable shape for covering the wound, then wetted, and applied to it. All discharge is absorbed by tbe dressing, and any bid odour 1b effectually destroyed. By dipping it into hot water, and covering it with oil silk, a convenient antiseptic poultice is formed, easily made, very light, and answering perfectly its purpose. It thus supersedes the use of lint, ointments, and linseed-meal or bread poultices. It is easily burnt—no small advantage in a crowded hospital, where bad-smelling applications are a fertile source of decrease if not quickly destroyed. Its simplicity of application saves much time and labour—a great consideration where the nursing staff is overtaxed, as inust always happen after serious engagements. Its cheapness is another great advantage. Carded oakum can be procured, we believe, for somethinglike ninepence a pound, about a fifth or a sixth of the cost of lint.

NEW ELECTRIC MACHINE.—Among the improvements of the "Influence or Electrophcr machine," invented by Herr Holtz, of Berlin, a new construction, by Professor Poggendorff, is especially noteworthy, This machine, we learn from the MonaUbmcht ef the Berlin Acadomy, has two rotating glass plates; and the conductors are placed in a convenient manner, especially adapted for demonstrations in lecture-rooms. It exceeds in efficiency all machines hitherto made, and Is manufactured by Borchhardt, of Berlin. For the purpose of cleaning the glass plates, Professor Poggendorff recommends rubbing with petroleum.

ALLOY FOR TAM-TAMS, OR GONGS, AND CYMBOLS.—From the Chemical New* wc have an account of the method now followed by Messrs. Riche and Champion, for making the sound-producing instruments alluded to, which consist of an alloy of 80 parts of copper and 20 of tin, which is hammered out with frequent annealing. Am alloy of 78 of copper and 22 of tin answers better, and can be rolled out. The process is similar to that in use in China, and is carried out at Paris by M. Cailar.

POISON PLANTS.—Mr. Worthington Smith records In the Journal' of Botany an instance of a fatal case of poisoning by oatiug the root of the water-dropwort, (Enanthe erocala, an umbelliferous plant, common in ditches and wet places. A carter at Staplehurst, in Kent, ate some of the roots whilst at work, supposing them to be the wild parsnip. In about an hour he became unconscious and convulsed, and death occurred in another half-hour, before medical aid could bo obtained. The man had fed his horse with roots of the same plant, and tho animal also expired about two hours after eating them. There is no doubt that the (Enanthe is a virulent poison, but it seems strange that tho horse as well as the man should not have rejected a -plant of so acrid and suspicious a flavour. Several wild Urnbellifers} are amongst the most dangerous of British plants.

THE NEW ACT ON FACTORIES AND WORKSHOPS.—The statute to amend and extend the Acts relating to factories and workshops was passed on the 9th August, iu reference to print-w>rks and bleaching and dyeing works, as also to amend the Acts on factories and workshops. There are two parts in the statue and three schedules. The first part relates to print-works and bleaching and dyeing works, and is to be construed as one with the Factory Act Extension Act, 1867; and after January 1st, 1872, the principal Act is to apply to print-works and bleaching and dyeing works. Certain regulations are to bo enforced on the 1st of January, 1871, in Turkey-red dyeing works. The second part of the statute relates to fruit and fish preserves. The schedule to the Factory Acts Extension Act, 1867, and the schedule in the Workshop Regulation Act, 1867, are to be construed as if they wero contained In each of those schedules. In the schedules the permanent modifications are set forth. In tho manufacture of preserves from fruit, and in the processes of preserving or curing fish, women may be employed, between the 1st of June and tbe 14th of December, for a period not exceeding 14 hours on auy one day.

ARTIFICIAL BUTTER.—Apparently astonishing as the idea is, the manufacture of artificial butter (beurrr, artificiel) is seriously contemplated in France. A Parisian contemporary states that M. Meye, of Paris, has taken out a patent for the manufacture of artiucinl butter, which according to the journal in question, is to be used as a substitute for that ordinarily exported to England and Russia. The description of the process is, that animal fat it subjected to great pressure, by which tho steai Ine is c xtraoted for making candles, or bougies: an oily material being at the same time obtained, the composition of which is identical with that of butter. The quention then arises, how is the taste of this " com

?option " to be rendered similar to that of butter? M. ;I< yo is quite equal to the occasion; he says that he subjects the animal oil to a succession of scientific processes and manipulations, the nature of which he explains at great length and with marvellous ingenuity. He tr-ice* the relations of unsophisticated grease to sophisticated fat, and crowns the edifice in rare style, bv showing the ■precise identity of each with what he calls' butter," and indulges in the most sanguine anticipations about the success of hi* " invention."

GELATIN.—It is said that an animal fed on gelatin dies rapidly of starvation and inanition. This is per

E

fectlytrue: but the fact is, that no simple substance given alone will support life. Tho most nutritive materials, such as albumen, fibrin, glu'.en—to nay nothing of such substances as starch or sugar— all equally fail to support life when given alone.

SCAVENGER BEETLES.—Scavenger beetles are to be seen wherever the presence of pntrescout and offensive matter affords opportunity for the display of their repulsive but most curious instincts; fastening on it with eagerness, severing it into lumps proportionate to their strength, and rolling it along in search of some place sufficiently soft in which to bury it, after having deposited their eggs in the centre. I had frequent opportunities, especially in traversing tho sandy jungles in the level plains to tho north of the island, of observing the unfailing appearance of these creatures Instantly on the dropping of horse-dung, or any other substance suitable for their purpose; although not one was visible but a moment before. Their approach on the wing is announced by a loud and joyous booming sound, as they dash in rapid circles in search of tho desired object^ led by their sense of smell, and evidently little assisted by tbe eye in shaping their course towards it. In these excursions they exhibit a strength of wing and sustained power of flight, such as is possessed by no other class of beetles with which I am acquainted, but which is obviously indispensable for the duo performance of tho useful functions they discharge.—Sir J. E. Tennent't Natural History of Ceylon.

HARMLESS SALAD HERBS.—It is a pity people, says the Pall Mall Gazette, in this country neglect to add to their diet by a more liberal use of harmless salad herbs which are now altogether wasted. Watercress is very well In its way, but is not everywhere accessible. It has lately been pointed out that our sorrel plant is a more delicate and agreeable variety than the wood sorrel so largely used at table iu France and Germany; that the common shepherd's purse and lady's smock may supply the place of the watercress.- that tho famous "barbe dos capucins" of the Paris restaurants is nothing more than tbe blanched shooU of the wild chicory, and that the "salade de chanoine" is our own neglected corn-salad. Our peasants, however, have vet to learn the art of making salad, and the wisdom which selects and dulcifies with oil the bitter qualities of such herbs as the endive or its proposed humbler substitutes, the avens, the bladder campion, and the tender shoots of the wild hop. This art mu-it descend to them from a more cultivated class, and iu nothing is the English middle-class table inoro generally deficient than in the wholesome, varied, and savoury salad which adds an attraction to the simplest fare, and gives something of freshness, if not of refinement, to tbe richest us to the poorest table.

SEA-CUCUMBERS. — The holothuria?, or Bea-cucumbers, may bo regarded in one light as soft seaurchins, and in another as approximating to the annelides or worms. Their suckers are similar to those of the true star-flshos and sea-urchins. Besides progression by means of thuse organs, they move, like annelidos, by the oxtensiou and contraction of their bodies. Tho month is surrounded by plumose tentucnla, the number of which, when they are complete, is always a multiple of five. They all have the power of changing their shapes in the strangest manner, sometimes elongating themselves like worms, sometimes contracting tho middle of their bodies, so as to give themselves the shape of an hour-glass, and then again blowing themselves up with water, so as to be perfectly globular. The great sea-cucumber Is the largest of all the known European species, and probably one of tho largest Cucumerue in the world, measuring, when at rest, fully one foot, and capable of extending itself to tbo length of three. Under the influence of terror, it dismembers Itself in the strangest manner. Having no arms or legs to throw off, like its relations the luidia and the brittle star, it simply disgorges its viscera, and manages to live without a stomach; no doubt a much greater feat than if it contrived to live without a head. According to the late Sir James Dal ye 11, the lost parts are capable of regeneration, evon if the process of disgorgement went so far as to leave but an empty sac behind. Considering the facility with which the sea-cucumber separates itself from its digestive organs, it is the more to be wondered how it tolerates the presence of a very remarkable parasite, a fish belonging to the genus Fiorasfer and about 6iu. long. This most impudent and intrusive comrade enters the mouth of the cucumber, and, as the stomach is too small for his reception, tears its sides, quartering himself without ceremony between the viscera and tho outer skin. The reason for choosing this strange abode is yet an enigma.—Dr. Hartwig's "Sea and it* Living Wonder*.*'

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AND

LHROR OF SCIENCE AND ART.

FRIDAY, SEPTEMBER 9, 1870.

STRACT OF A PAPER BY DR. ZOLLNER, 3N THE TEMPERATURE AND PHYSICAL CONDITION OF THE SUN.*

Bv Richard A. Proctok, B.A., F.RA.S.

Author of " Other Worlds than Ours," &c.

» PAPER of considerable scientific interest

V has recently been contributed by Dr.

jUner to the subject of the •' Snn's Physical

ondition." The following paragraphs may be

igarded as an abstract (and, in some places, a

-anslation) of this paper, which would be too

ing to appear in full in these columns. It has

eemed best to give the calculations in full,

>ecause, though too complex to be understood by

til our readers, they will be acceptable to many.

The results to which they lead can hardly fail to

>e generally interesting.

The solar prominences may be divided into two characteristic forms—those resembling clouds or smoke, and those which present all the features of mighty eruptions. Sometimes one, sometimes another form will be most prevalent over a particular region of the sun's surface. When we study prominences of the latter form attentively we can scarcely doubt that they are really due to eruptions of glowing hydrogen. Now, if we assume that these eruptions are caused by an excessive pressure exerted on the hydrogen while yet beneath the son's surface (and no other theory seems available, unless we abandon altogether the guidance of recognized analogies), wo must suppose that there is a stratum partitioning off the inner masses of hydrogen from the hydrogen atmosphere outside the photosphere.

The conception of such a stratum has been forced even on those who hold, like Respighi, that electricity causes these signs of eruptive action. But, adhering to the simpler and more natural supposition of a difference of pressure, we have the means of answering some very important questions respecting the temperature and physical condition of the sun.

We have in the case of perfect gases :— 1st. The law of Mariotte and Gay-Lussac. •2nd. The constant ratio which subsists between the specific heat at constant volume and the specific heat at constant pressure.

This ratio, when once determined by recognised methods, for any gas, must be regarded as unchangeable—like the atomic weight of substances and other such constants. In order to deal with the eruptive prominences, as instances of the outflow of a gas from one space into another, we assume, for the sake of simplicity, that there is a constant pressure in both spaces, and neither gain nor loss of lieat.f

Put A = the heat-equivalent of work (the reciprocal

of Joule's equivalent), ti = velocity with which the gas flows from the opening. = force of gravity at the sun's surface. = the constant ratio—

tspecific heat of gas when pressure is constautT _ its specific heat when volume is constant J e = the ratio of the specific heat of the gas.

to that of water. t i = the absolute temperature of the gas in the

inner space. f« = the absolute temperature of the outflowing gas in the plane of the vent. = the pressure of the gas in the inner space. = the pressure of the gas in the plane of the

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Assume further that

a, = the mean height of the barometer in metres.

(i = density of the gas, at the temperature of melting ice, and at the pressure corresponding to the barometric height a, at the earth's surface.

a = density of the gas in the inner space mentioned above, under pressure />, and at the absolute temperature fj.

a = the coefficient of expansion of the gas for 1° Centigrade. From the law of Mariotte and Gay-Lussac we

have the following relation:—

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(3)

p Pi "~ a, a' tt

The pressure js„ may be considered as equivalent to the pressure of the outer solar atmosphere

at its base. Next let

h a certain height above the base.

ph = the pressure at this height.

t = the absolute temperature of the solar atmosphere assumed to be constant.

r = radius of the stratum separating the inner from the outer space.

p( = specific gravity of mercury at the temperature of melting ice.

q = force of gravity at the earth's surface.

a = mean height of the barometer.

a = the density of our atmosphere at the temperature of melting ice, and under the influence of gravity </, and atmospheric pressure a,. Then we have, from known laws, the formula , /Pa\ P 9 rh_

loe<K¥j

* Dr. Zi.illx.6r, '* Ueber die Temperatur utul phytitehe Betc'iaffetiltcit >Ur Sonne."

i There would commonly be a great departure from both conditions, but a little consideration will show thnt fur the purpose of tbe present inquiry these conditions are necessary, while the results obtained may be traced as indicating the limits of temperature and pres sore in the cases considered.—R. A. P.

w

Pi 9, a, a t (r + h) And in order to associate this formula with the preceding, we must assume

1. That the main portion of the solar atmosphere consists of the same gas which flows out to form the eruptive prominences.

2. That the absolute temperature of the solar atmosphere t is equal to the temperature ta at the level of the vent.'

We have thus the p of formula (4) identical with the p in (3), and t = t*.

It is now necessary to remodel and simplify our equations.

Put H = height which a body would reach if projected with velocity t> from the sun's surface. We have then, taking into account the diminution of gravity,

r' = 2ffHr-TH

r* r H

or % " r + H Substituting in equation (1) we get HA

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And eliminating, we deduce

J bph A+jV »■

° +« V l /

From this equation, if any three of the four quantities a, />;„ A, and t can be determined by observation, or shown to lie within certain limits, the fonrth, or the limits between which the fourth lies, can be calculated. But the limiting values of the quantities o-, ph, and h can be determined

* Zollner infers the justice of the second supposition, from the fact that the bases of the prominences seem about as bright as the chromosphere. But though there is very little apparent difference, yet the photographs taken daring the American eclipse indicate the existence of a real difference of brightness. For tho edge of the lunar disc is eaten awav, Bo to speak, at the bases of the prominences. It is true that Dr. Gould considers this is due to specular reflection at the moon's surface, but there Is every reason to believe that Dr. Curtis is right in regarding the peculiarity as "a phenomenon of the dark room "—i. t. a purely photographic effect. However, it is important to remark that Dr. Zi.Uner's results are not importantly affected by a change hi the assumptions—It, A, P.

partly by spectroscopic analysis, partly by other modes of observation. Thus we can estimate the limits of t (that is, the temperature of the hydrogen atmosphere). These limits substituted in I. give (from the known value of H) limits for the inner temperature tt; and thence, by means of III. and IV., we can estimate the limits of the pressures p, and />„.

The lowest value for I is obviously 0. So that

the minimum value of I, is

r H A

1 = «c(r+ H)

As the density of the solar atmosphere at moderate distances from the sun has been shown to be almost evanescent, we may, for simplicity, assume that H is equal to the mean height of the eruption prominences.*

Many prominences 3' in height have bocn seen. But let us take H at only 1-5'.

The heat-equivalent A may be put as --

(taking the metre as unit of length, and centigrade degrees of heat). According to Regnault's latest results, K C is 3-409 for hydrogen; and according to Dulong, K for hydrogen is 1-411.

Zollner here enters into a discussion of the probable value of r. From a consideration of facts already made public, he comes to the conclusion that the stratum which separates the inferior gaseous masses from the outer atmosphere must lie beneath the photosphere—which is the stratum where, through increased pressure, tho spectrum of the hydrogen atmosphere becomes continuous. Taking the nuclei of spots as probably correspouding to the lower stratum, and the depth of such nuclei as 8", he suggests as a probable value for r,

r =-. R-8", where R = sun's radius, or putting R = 16' at the Sun's mean distance, r = 16' 52" Taking Hansen's value of the sun's mean parallax (8"-9159) we have

r = 680,930,000 metres 8" = 5,722,000 ,, and H = 64;870,000 „

Substituting these valueB in the expression for U (when t = 0) we find

t = 40,690° Centigrade. Giving H a value twice as great as this, (i = 74,940° Centigrade. These are minimnm values of ( for the assumed heights 1-5' and 3', respectively, to which the eruptive prominences are supposed to reach. Zollner here pauses to inquire whether the ascent of the prominence matter may not be due to the same principle which makes heated air ascend in a chimney, rather than to projection through a resisting medium. Of course, in the former caso, his whole argument would fall to the ground, because instead of compressed hydrogen rushing out through an opening into a relatively rare atmosphere, there would be relatively rare hydrogen rising by reason of its lightness through a denser atmosphere. He rejects this conclusion, however, on account of the enormous velocity with which the prominences havo boon observed to rise—whether their formation is measured directly and timed, or by spectroscopic observations depending on the change of wavelength of light from prominence-matter moving towards or from the eye. He finds that the velocity of projectionf neSessary to hurl matter

• In other words, Zullner here assumes that the height to which the erupted gas is oluemr.il to be projected through the resisting atmosphere, i-i not vory fur less than that to which it would be projected by a liko force, in vacuo. Owing to the probable compression of tho erupted gas, the supposition is admissible, at least for the purpose of the above inquiry. The maximum rather than the mean height of the prominences should be taken, however; because obviously we eannot suppose all the observed prominencos to be really projected from tho edsie of the disc, many must be jortshorUned portions of prominences.—R. A. P.

+ I am led to notice here the amazing conception of Mr. Mattieu Williams-s, that matter is projected from the sun far beyond tho orbits of Mercury and Venus, and even beyond the orbit of the earth and Mars. It would not be difficult to show the enormous improbability that any explosive forces exerted by tho sun could produce such results; but setting aside all such reasoning, and giving all due weight to the possibility of a tangential motion being imparted as well as a radial motion, it is certain that any bodies so projected must return to the point of projection. Setting aside the relatively minute effects of planetary perturbations, any body projected from the eun, even tangeutially to his surface, must continue to have the place from which it was projected as a point °I its orbit. Yet Mr. Williams gets his projectiles altogether free of their source, and so makes them act all manner of unexpected parts. They bocome meteors, though the orbits of most mettors i»is to the observed height attained by the prominences corresponds well with these observed velocities, whereas no such velocities conld be expected if the hydrogen merely rose by reason of its relative lightness.

To resume, then, the consideration of the xniting values of«,-.

We can deduce a limit for I by discussing equation V. It may be readily shown that <r cannot exceed the mean specific gravity of the sun, or 1-46. Giving a this value in V.; to a the value 40,690° deduced above, to h the value 8" at the suii's distance, expressed in metres, aud to p the limiting values 0-500""" and 0-050"TM deduced by Wullner, we get the following values for t. Whenj),, = 0o00""» t = 29,500° „ ph = 0-050""" t = 26,000°

The mean value 27,700° will be adopted.
Now, differentiating equation V. with respect to
da
I, tlie differential coefficient , , is found to be

negative,*—that is, as a diminishes t increases. Therefore since the above value of I has been obtained by giving a its maximum value, 27,700' is the minimum value of t. It corresponds to a tame of 0'ltiD •'■■" for pi,; which value will accordingly be adopted in the following computations.

It may be noted in passing that the high values obtained for (, show that iron can only be present in the form of vupour in the solar atmosphere.

Taking

t = 27,700° we get f, = 68,400°.

Putting these values in II., we find

— = 22-1;

Pa

that is, the pressure within the space from
which the prominences flow is 22-1 times greater
than the pressure at the surface of the fluid layer
supposed to separate the compressed gas from the
gas outside.
Again from formula IV. (putting ft as 8") we get

— = 766,000,
pi>

«• the pressure on the fluid surface of the sun is
(on the assumptions we have made) 766,000
tunes greater than the pressure at the height
where the hydrogen spectrum begins to be con-
tinuous owing to pressure.
Putting

ph 0-180"1 we get
pa = 184,000 atmospheres,
sndps = 4,070,000 „

Beckoning the depth at which in a fluid of specific gravity 1-46 the mere hydrostatic pressure would be pi, we find that this depth is 139 geographical miles, or about 650 English miles ; that is at about l-658th of the sun's semi-diameter, or 1-46 seconds of arc below the limb.

Even taking the case of a dense atmospheric ihell of hydrogen, we find that for a temperature of t»,4CKr ((,) we have a pressure of 4,070,000 atmospheres (/>,), only 27" beneath the visible limb

nowhere Dear the sun; they become asteroids—the gap el 157 millions of miles now separating the nearest asteroidsl orbit from the sun being seemingly as of no moment whatever. It cannot be too plainly insisted on '.this projection theory having so often been a stumblingblock) that while the bursting of a planet into fragafteuts which eventually pursue iudepeudent orbits, or the projection of a body from a planet so that that body eventually follows an independent orbit, may be conceived (at any rate) as possibilities, thu central body of a system cannot possibly project bodies in such sort ttiai they can travel in independent orbits around him. I do not say that the two former suppositions are one whit more acceptable than the latter. There are abundant reasons for doubting whether a planet could burst in the way according to which the f ormatiou of the asteroidal zone has been explained. Nor have we any nason for believing that eruptive action on any planet could expel a body far enough to free it from the control A the planet, and enable it to follow an independent orbit around the sun. But we can conoeive such things, precisely as we can conceive (without believing) that tho sun might project a body beyond the orbit oi Neptune. What we cannot oonceive, without forgetting the essential principles of dynamics and the law of rravity itself, is that a body projected to any distance trom the sun could ever be found travelling on au orbit Tthich at no point approaches the sun.—ft. A. P.

* All the quantities dealt with in equation V. arc positive. Differentiating we get—

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of the solar disc, or about l-86th of the visible solar semi-diameter.

This shows how quickly the pressure must increase within the sun's globe, and justifies the supposition that, even at the enormous relative temperature undoubtedly existing, the so-called permanent gases—hydrogen, oxygen, and nitrogen for instonoe—can only exist in tho form of glowing fluids*

A very surprising result follows, when we consider in turn the case of a nitrogen or oxygen atmosphere, having the same specific gravity and temperature as the hydrogen atmosphere (at the level of tho dividing stratum), and reckon tho pressure existing in each atmosphere at that height where the hydrogen spectrum begins to become continuous. Taking the pressure of the three atmospheres asoqnal to 184,000 atmospheres, at a depth of 8" beneath the sun's visible limb, we get (with the temperature ( = 27,700°)

For hydrogen pn = 180 millimetres
323
„ nitrogen p = ^

„ oxygen = 1Q

So that the quantity of these two gases, on the above suppositions, may be regarded as absolutely evanescent where the spectrum of hydrogen begins to become continuous. This would also clearly be the case if wo supposed the weight of the two atmospheres many millions of times greater, though it would only require that the weight of given quantities of nitrogen and oxygen should be 14 and 16 times (respectively) less than it actually is to cause these two gases, under the assumed conditions, to have a density coincident with that of the hydrogen at the sun's visible limb. It is easy to calculate, by means of formula III., and thu known specific gravity of oxygen and nitrogen, how great the weight of each atmosphere must be in order that the density at a depth of 8" beneath the sun's limb may reach its maximum value 1-46. We thus find that the weight of the oxygen atmosphere can amount to but 056, that of the nitrogen atmosphere to but 0-64, where the weight of the hydrogen atmosphere is represented by unity.

It appears, then, that putting out of question the existence of disturbances in the solar atmosphere, the rays sent from the hydrogen which exists at so great a pressure as to give a continuous spectrum, can pass through so small a quantity only of glowing oxygen and nitrogeu, that the absorption would be imperceptible, and therefore (as we know to be the case) the spectroscope could afford no sign of the presence of oxygen or nitrogen in the solar atmosphere.

However, as the absorption-lines of iron and other metallic elements show that we might (were mere quantity in question) expect to find traces of the presence of oxygen aud nitrogen, it is necessary to add to the above considerations the circumstance that the permanent guses have but slight emissive power compared with the vapours of ordinarily solid bodies. The absorptive power being proportional to the emissive, we can understand (or at least it is no longer altogether perplexing) that tho lines of substances whose vapourdensity must be considerably greater than that of oxygen or nitrogen should be seen in the solar spectrum, which yet shows no oxygen or nitrogen lines.

Zollner then presents the following propositions

* Against the supposition that such gases as oxygen, hydrogen, and nitrogen can under any circumstances assume the liquid or solid condition, it has been objected that all tho gases which can be liquefied exhibit under pressure a departure from the law that elasticity varies with density; whereas other gases exhibit no appreciable departure from that law, even under the greatest pressure yet applied. Bat it is known that in the cases of the tiquefiable gases the departure from the law increases in a marked manner as liquefaction is approached, it follows, therefore, that the appreciable departure from the law undor ordinary pressures only proves that the pressure necessary to produce liquefaction bears a measurable ratio to ordinary pressures. It gives us promise, that is, of being able to liquefy such gases,—a promise justified by the event; and conversely, all that is shown by the absence of any such departure from law in the case of other gases, is that tho pressure necessary to liquefy them vastly exceeds any yet applied. The fact is sufficient to discourage any hopes we might have formed that such gases conld be liquefied by us; but by no means proves that they oanuot, under any circumstances, assume the liquid or even the solid form. As a parallel instance, we can conoeivo that no means would avail to produce variations of temperature many degrees on either side of the freezing point. In this case cortain peculiarities noticed in the expansion of water with such variations of temperaturo mi^ht lead the experimenter to infer that iron or gold, which exhibit uu such peculiarities under changes of temperature, cannot be liquefied. The conclusion would be orroneous, as probably is the abuve inference resecting gases.—K. A. P.

os flowing either directly or indirectly from hicalculations :—

1. We cannot conclude from tie want «J captain lines in the spectrum of a self -luuiinoai siir that the corresponding elements are necaaxray absent from its substance.

2. The layer in which the reversal of tie spectrum takes place is different fur eacj »-s.i_ stance, and lies nearer to the centre oi tit ssr according as the vapour-density is greater i&j as the emissive power is less.

3. Under otherwise equal Citcuesiieos t&Ii stratum lies nearer to the centre athe hHaatv of gravity is greater.*

4. The distances separating the wasl-strata of given substances, from each other a well as from the centre, u;o greater as the tesjtatare is greater.

5. Under otherwise equal rirrnrrrTii the spectra of different stars are richer in kn tho lower the temperature and the greater lb safes of the star.

6. The difference in the intensity of aSetsL dark lines in the spectrnm of the Bud ana s4a stars does not depend only on the differfcoj tetween the absorptive powers (of the oomswaing elements), but also on the different depOe tt which the reversal of the spectra In qmxsc takes place.

Zollner closes his paper by dwelling on is great importance of distinguishing between ttrespective influences of temperature and pressor on the spectra of gases and vapours. Me poinv out the possibility that, when examining th> spectra of electric discharges through raren<-J gasea, the pressure might be kept constant by an emission of gas before each discharge; and thus the means be obtained of determining the effect of temperature alone. He notes, however, that the results presented above could not be importan tlv affected by any change in the estimated pressure at which the hydrogen-spectrum becomes continuous, since, corresponding to limits of pressure proportioned as 1 to 10, the limits of the resulting temperature are proportioned as 1 to \ ^ only.

THE MICBOSCOPE.—HOW TO CHOOSE AND HOW TO U8E IT.

(Continued from page 532.)

WE do not propose any rigid order of sequence in our further notes upon the microscope, but we will endeavour to take the several pieces of apparatus as they appear to stand in the order of desirability. To this end we will allow ourselves considerable latitude in our interpretation of the word accessory, and include therein all apparatus not supplied with an ordinary student's microscope. Aud the question we will set ourselves to answer shall be, What extra apparatus, and in what order, would you recommend to a student? I think we may place the polariscope in the first place, partly because whether we do or no the student is tolerably sure to do so, and partly because in proper hands it is an invaluable instrument of lesearch. In the form most corxunouly applied to the microscope the polariscope consists of two parts—the polarizer and the analyzer. These are precisely similar in construction and may (provided their fittings admit) be interchanged at pleasure. The polarizer and also the analyzer may consist of a rhomb of Iceland spar, or a thin plate of tourmaline or of herepathite (sometimes called artificial tourmaline). On account of its cheapness and freedom from colour, as well as its greater freedom from danger of accidental damage, the Iceland spar is generally used, and in the form known as Nicol's prism. Instead of t\ie spar as a polarizer thin plates of glass may be used, but as they do not permit of rotation readily their disadvantages are great. The polarizer tits into the stage from beneath, and it is well if a bayonet catch or some simple appliance be arranged to hold it firmly in position. The prisms are fitted into a collar, which rotates easily by a milled head, unless the stage be an elaborate mechanical affair, when special arrangements are made to receive the prisms. The analyzer may be fitted either immediately above the objective, in which position there is some Iobs of definition, or above the eye-piece, in which case there is a considerable loss of field. In general work the former position is the best. In any caso the prisms should bo made capable of rotation with exact centering. If it bo desired to carry the

* If r he tho radius, M the specific gravity of a star suppo.-cd uniform, the intensitv of gravity at its surface varies as . r.—K. A. P.

analyzer above tho eye-piece either tourmaline or herepathite should in all cases be preferred to Iceland spar. The price of a good polnriscope and fittings adapted for a student's microscope varies from 30s. upwards. A selenite of some kind is almost a necessary accompaniment of a polariscope. Sometimes a thin film (to give red, yellow, or blue, according to fancy) is fixed in a collar and made to rotate with the polarizer. But this is a barbarous arrangement. In all cases the film of selenite should be mounted on a glass or in a brass slide, and some means adopted by which it may be removed from the field at pleasure without disturbing the object. It is also convenient to have three films giving different colours set in the same frame, their axes of polarization being in the same plane, that a series may be tried without trouble. A very efficient selenite stage has been described in a recent number of this journal, the price, however, is past a joke to most.

Having obtained the polariscope, the student, if he be a worker and use his J" much, will find that he needs a steady and pure light such as is thrown upon the field by what is known as the achromatic condenser. This is optically little other than an objective of J" or J" focus made capable of approximation to the lower surface of the slide under view. It is a somewhat expensive piece of apparatus, costing from 30s. upwards. Mr. Brooke recommends a Kellner eyepiece in the place of the usual short focus objective. In any case, the student may remember that he can always use his f' objective as a condenser for his I", and so on. In this case he need only purchase fittings which need not be very costly. Mr. Collins has, within the last few years, introduced a new condenser, which he calls the Webster. From what we hear of it it seems a most efficient affair, especially in connection with his adjusting diaphragm.

Mr. Swift has also introduced a popular and complete condenser, of which, as it is a perfect multum in parvo, we give a somewhat detailed description:— " It comprises an achromatic condenser of 90° angular aperture (this is suitable for all objectives from the 2in. to one-fifth), tinted ditto for correcting the yellow rays of artificial light, rack-and-pinion motion for focussing, a most effective spot lens, parabola, large diaphragm, carrying rotating cap—into this cap are fitted three discs for test stops—small diaphragm of apertures, polarizing prism, selenite diaphragm with two selenite films and clear aperture, and oblique light shutter for illumination with low powers.

•' The optical combination is made with an angle of 90° or 120°, the former for objectives to the one-fifth of an inch, and the latter for those of Jin. There are two 'field lenses,' one of uucoloured glass for use with daylight, the other of a blue tint of sufficient depth to correct the yellow rays from an ordinary paraffin lamp, so desirable in long-continued study with artificial light. The condenser is attached to the stage of the microscope and held in its place by a bayonet catch, and the focus adjusted by a milled head with rack and pinion. The sliding frame has an open space and two spots, made central by a spring catch, and of a suitable size for the 2in. and lin. objectives, for dark ground illumination. The advantage of having these stops immediately beneath the lenses is very apparent in the much darker field that is obtained; their value also in this position is greatly enhanced by being able to use them in conjunction with the polarizing prism and selenite films, thereby bringing out in bold relief such balsam-mounted objects as the palates of the Mollusea. Tho rotating cap can be removed with the greatest facility and other stops substituted in the place of those (three) supplied with the instrument, as the nature of the test under examination may require. The use of the oblique light shutter and the small diaphragm of apertures are too obvious to require explanation. "With the exception of the frame of stops all the appliances are carried on the large diaphragm, and are brought into position by simply rotating that wheel until stopped by its spring catch.

"If required, a parabola for dark ground illumination with high powers is made to fit into the place of the optical combination."

The writer of the paper published in the "Transactions of the Royal Microscopical Society" (new series, No. 51, July, 1868, p. 110), claims for this condenser the following advantages! viz.,—

"1. It can be used with marked advantage with objectives from 2in. to Jin.

"2. The remarkable daylight softness produced by the tinted field lens when used with

artificial light; also dispensing with the use of blue lamp chimneys.

"3. It is a very effective spot lens, and dark ground illumination with polarized light.

"4. An almost indispensable requisite for polarized light when using high powers with the object mounted in fluid.

"5. And not least important, the ease and rapidity with which the changes from ordinary to oblique, and plain or coloured polarized light with the other combinations I have named, can be made."

Professor Beale recommends Lealand's Gillett condenser, but we have not space here to enter into a discussion of the respective merits of different makers of apparatus. It must suffice for us that we glance at every important accessory and give enough details to enable our readers to ask for what they want.

Our students will now probably seek to increase his "power." He can do this by the addition of a draw-tube to his instrument at the cost of a few shillings, but unless his lenses are very good this plan is not to be recommended. The drawtube in connection with an erector (for rectifying the inversion produced in the apparent position and movement of objects when they are viewed through a compound mirror) is, however, most useful, as it enables the observer to get a range of from four linear to 100 linear with an 8-10ths objective. Or he may add a C, D, E, and F eyepiece to his instrument, gaining in power approximately 1, 2, 4, &c. But the better plan is to got a C and then to add to his objectives first a J", l-12th, l-20th, or l-25th to l-50th, as he is able. But he will probably stop at l-12th, and seek other accessories.

The first thing will most likely be a mechanical stage,—that is, a stage capable of movement in all directions by means of milled heads. Their forms are legion, every maker appearing to have his special type. Let our student see that whatever form he purchases possesses tho power of concentric rotation and that the amount of movement can be read off accurately by a scale. He will now want a Maltwood's finder, a very simple and inexpensive arrangement for enabling a person to register the position of any object in the field. It consists merely of a slip of glass whereon is photographed a series of numbers in squares thus—

A B, to a focus at F, can form no part of the largest pencil of light admitted by any of the object-glasses, and represented by G F H; bol

[table]
[table]

A little "stop "is placed at one end of the stage against which the end of the slide is placed, when any object of special interest is found the slide containing the object has to be removed and tho " finder " placed in its stead and the numbers in the field read off and registered. It is obvious that the reverse process is simply to work the stage until the finder now comes into the field, remove the finder and place the object slide on the stage when tho desired object will be found in the field. The price of a Maltwood varies from 5s. upwards. Substitutes have been proposed for the Maltwood, but the latter is so simple and inexpensive that we need not trouble to examine these.

Every observer ought to draw and measure his objects, but as this subject will come before us in another paper we will simply mention the names of the apparatus commonly used for these purposes. For measuring, either the stage micrometer or the eye-piece micrometer is used. For drawing, a camera lucida (a prism properly set), a neutral tint reflector, known as Beale's neutral tint, or a small disc of steel.

We have omitted to mention the spot lens a

simple and inexpensive, but very efficient means for procuring what is known as the dark ground illumination. This apparatus consists merely of a lens of moderate focus with a spot of black paper on its centre to stop out all rays except those which pass through the periphery and converge at so oblique an angle upon the object that were it not for its refraction they would not enter the object-glass at all. Certain objects, such as diatoms, are seen by this mode of illumination brilliantly illuminated on a black ground. Mr. Wenham has introduced what is known as Wenham's paraboloid reflector. It may be easily understood by reference to Fig. which represents it in section ABC, and shows that the rays of light, r r' r", entering perpendicularly at its surface, C, and then reflected by its parabolic surface,

an object placed at F will interrupt the rays ami be strongly illuminated. A stop at S prevents any light passing through direct from the minor, which is more adapted for high powers than the spot lens. Indeed, the latter cannot be turfed with what are properly called high powers.

The celebrated diatom prism which has recently caused such a sensation does not require any explanation beyond this, that it is a rightangled prism fitted either to the stage or on a stand, and that its great advantages are that it enables us to obtain a beam of parallel rays of such intensity and in such direction as we require. The goniometer for measuring the angles of crystals we need not describe, as a well-constructed mechanical stage will serve most of its purposes. Neither will we do more than mention Amici's prism for obtaining oblique light; and several other accessories but seldom used must be entirely passed over.

For " opaque " illumination we may use either the parabolic illuminator known as Crouch's but made also by several makers, or the side re flector, or with very high powers Messrs. Beck'patent illuminator, whereby the objective is made its own illuminator by a most ingenious contrivance. The cost of all these is comparatively trifling. It strikes us very forcibly that we have spoken of nearly all accessories appertaining to the microscope proper. In our next we will hurry through such things as processes and mounting materials. If any reader wishes further information respecting any apparatus mentioned in this paper, perhaps the editor will allow him to ask any question he chooses. H. P.

P.S.—In my last article I appear by some oversight to have said that a fair English 1" objective could be purchased for 50s.; I should have said 30s. I am pleased to learn that a London maker is selling a tolerably fair 1" for the astonishingly low price of 12s. As my caution to our readers appears to have been misunderstood by some of the makers, I may, perhaps, be allowed to say that nothing was further from my intention than to reflect upon that most painstaking section; I merely wished to advise our readers to make as much as possible and to buy as little as possible. It is only just to our opticians to say that they are always willing to help the amateur to save his pocket if he will trust himself to them.

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