results were obtained by the correction of his error, how superior must the microscope be in which the concentric adjustment of its lenses is effected! While opticians, indeed, confine themselves to the use of only two kinds of glass, of different refractive and dispersive powers, we can hardly expect much improvement in the microscope, unless by the substitution of achromatic lenses in the eye-piece, and by an infallible method of centring each lens, and each group of lenses, in the instrument. The successful application of two pairs of adjusting screws to each of six lenses, and also to those of the eye-piece, may be a difficult task, but it is not beyond the powers of mechanism. It is very obvious that Dr. Wollaston's method of examining the centring of a triple object-glass is wholly inapplicable to the object-glass of a microscope. In submitting to examination an object-glass made by a distinguished optician, it was necessary to use a microscopic picture of the sun, and to examine the position of its images as reflected from the various surfaces of the lenses by means of a microscope, the object-glass of which was brought in contact with the outer lens of the object-glass to be examined. By separating the two object-glasses, I observed in succession a series of twenty-four images appearing and disappearing in succession. These images occupied different parts of the field, and I could not succeed by the most careful adjustment of the apparatus employed in placing them in the same axis. These images had various sizes, and were in various states of colour, some highly coloured, and some purely white. They had also various sizes, many with fine planetary discs, of different magnitudes; some like the smallest fixed stars which it was difficult to descry, and almost all of them exhibiting the most beautiful concentric diffracted rings when put out of focus. Two or three images often appeared in the same part of the field, in immediate succession, while similar pairs arose at a distance from each other. Although I often succeeded in uniting two or more of these images, yet the effect of this was to place others at a greater distance; and I had no hesitation in coming to the conclusion, that the lenses of the object-glass which produced these images were imperfectly centred. Having had occasion to see at the Paris Exposition, and more recently at Florence, the superior performance of Prof. Amici's microscopes, I cannot omit the present opportunity of urging philosophers and opticians, as I have often done, to correct the colours of the secondary spectrum by fluids or solids of different dispersive powers. Prof. Amici has done this. In his object-glasses, Nos. 1 and 2, of low powers, he employs five different refractive and dispersive substances. In his powers Nos. 3, 4, and 5, he employs five such substances; and in his highest power, No. 6, he employs six. In recommending, as I have often had occasion to do, the employment of diamond and other gems in the construction of compound as well as simple microscopes, I have been met with the objection that they are too expensive for such a purpose, and they certainly are for instruments intended merely to instruct and amuse; but if we desire to make great discoveries, to unfold secrets yet hid in the cells of plants and animals, we must not grudge even a few diamonds to reveal them. If Mr. Cooper and Sir James South have given a couple of thousand pounds for a refracting telescope, in order to study what have been miscalled "dots" and "lumps" of light on the sky; and if Lord Rosse has expended far greater sums on a reflecting telescope for analysing what have been called "sparks of mud and vapour" encumbering the azure purity of the heavens, why should not other philosophers open their purse, if they have one, and other noblemen sacrifice some of their household jewels to resolve the microscopic structures of our own real world;-to unravel mysteries most interesting to man; and disclose secrets which the Almighty must have intended that we should know? On a new Polarizer, resulting from a Modification of the Prism of Nicol. By M. LEON FOUCAULT, Paris. When it is proposed to polarize in a complete manner a pencil of white light, the best means is to recur to the prism of Nicol; but if a pencil of a certain volume is to be acted on,-from four to five centimetres diameter, for example,-Nicol's prism becomes expensive and difficult to realize, on account of the scarcity of the beautiful specimens of the spar of Iceland. The cut adopted for the construction of the prism of Nicol entails necessarily a great cost of material. To have the prism entire, a crystal of spar is required, whose longitudinal ridges are at least equal to three times one of the equal sides which terminate the bases. The piece is then cut from angle to obtuse angle by an inclined plane of 38° on the plane of their bases, and perpendicular to that of their smaller diagonals. The two surfaces thus obtained are polished and glued together with balsam of Canada, when a parallelopiped thus prepared is placed on a bottom uniformly lighted. On looking through the piece, a field of polarization is seen contained between two curved bands,-one red, the other blue, which correspond with the direction of the limits according to which the ordinary and extraordinary ray are transmitted. These bands comprise an angular space of 32°, which makes Nicol's prism an analyser, applicable in all cases where the inclination of the ray, which it is desired to observe simultaneously, does not exceed 32°. But this angular extent of the field of polarization, which is sought for in the prism of Nicol, considered as an analyser, no longer presents the same interest when the apparatus is to fulfil the part of a simple polarizer; for then the action desired to be produced acts only, in general, on a pencil of light nearly parallel. So that there will be an advantage, in similar circumstances, in increasing the extent of the transverse dimension of the prism, even when the consequence would be a certain reduction in the extent of the angular field of polarization. Reflecting on the data of the question, I have in effect discovered, that we can modify the prism of Nicol in its cut, so as to diminish considerably the length without injury to its character of polarizer. I take then a parallelopiped of spar, whose longitudinal ridges equal only five quarters of one of the sides of the base. An inclined section of 59° on the plane of the bases, and the new surfaces, being polished, I put the two pieces in their natural position without fastening them, taking care to preserve between the new surfaces a little space, where the air penetrates, and which, with the proper incidence, determines the entire reflexion of the ordinary ray. Looking through a rhomb thus prepared-in other respects mounted like a prism of Nicol there is still discovered an angular field of polarization; but the index of refraction of air being considerably below those of the two rays propagated by the spar, complete polarization only takes place in an extent of 8°, and the field it presents is found comprised between two red bands. The new combination then does not fulfil the conditions necessary to the formation of a good analyser; but when it is only required to polarize a pencil of solar light, whose extreme rays have an inclination but of half a degree, the prism, with the thin stratum of air and its eight degrees of field, more than suffices to polarize all the elements of such a pencil. This kind of polarizer is even in some respects preferable to the prism of Nicol, provided that the reflexion of the ordinary ray takes place under an incidence which sends it back almost normally to the intersection of its two lateral faces; this ray has no tendency to issue by the base and confound itself, as in Nicol's prism, with the extraordinary ray. Also, when the material of spar is very pure, it accomplishes the extinction of the pencil produced by an analyser in a complete manner on the whole extent of the transmitted pencil. It is likely that in cases where the prism of Nicol is employed as a polarizer, the new form will be preferred, since it produces an effect more complete, at the same time economizing nearly two-thirds of the mass of spar. On a Telescope Speculum of Silvered Glass. The astronomical refractor, compared with the reflecting telescope of the same dimensions, has always had the advantage of giving more light; the pencil of rays which fall on the object-glass passes through it for the most part, and is employed almost entirely in the formation of the image at the focus; while on the metal mirror a part only of the light is reflected in a converging pencil, which loses still more by a second reflexion being brought back towards the observer. However, as the reflecting telescope is essentially free from aberration of refrangibility, as the purity of its images depends only on the perfection of a single surface, as with regard to focal length it possesses a greater diameter than the refracting telescope, and thus partly regains the light wasted by reflexions-some observers continue to give it the preference, chiefly in England, over the refracting telescope for the examina tion of celestial objects. It is certain that at this moment, and despite the multiplied improvements in the manufacture of large glasses, the most powerful instrument directed towards the heavens is a telescope with a metal speculum. The telescope of Lord Rosse is 6 feet English in diameter, and its focal distance is 55 feet. Possibly the reflecting instruments would have gained the superiority, could the metal take as durable a polish-could it be as well worked as the glass, and were it not heavier. Placing thus in parallelism the two sorts of instruments, and discussing their respective qualities and defects, I finished by conceiving that the telescope with a glass would possess every advantage, if the mirror being once shaped and polished we could communicate to it the metallic brilliancy, in order to obtain from it images as luminous as those of the refracting telescopes. This thought, which at first appeared a fiction of imagination, was soon converted into a satisfactory reality. The glass being cut by an experienced optician, and thoroughly polished, is ready to be covered by Drayton's process with a very thin uniform coating of silver. This metallic coating, which when taken out of the bath in which it is formed is dull and dark, is easily brightened by rubbing with a skin lightly tinged wth oxide of iron, and acquires in a short time a very brilliant lustre. By this operation the surface of the glass is wholly of metal, and becomes vividly reflective, not exhibiting under severest tests the slightest alteration in form. To procure a disc of glass with concave surface perfectly finished, I applied to Mr. Secretan, who had the kindness to provide for me a clever workman. On the other hand, to be able to obtain a deposit of silver, I had recourse to the owners of the English patent, M. Power and M. Robert, who actually work the process in France, and who furnished me with the silvery solution, giving at the same time the fullest instructions how I might best succeed. My mirror being silvered, and having acquired a polish of steel, I formed a telescope of it of ten centimetres diameter and fifty centimetres focal length. This little instrument supports well the eye-glass, which magnifies 200 times, and compared with the reflecting telescope of one metre, gives a very sensibly superior effect. Wishing to learn the proportion of light usefully reflected by the layer of silver deposited on the glass, and afterwards polished, or, at least, to compare the intensity of a pencil of rays reflected by a surface thus prepared with that of one transmitted by an equal surface from the object-glass of a refracting telescope, I accomplished the matter without difficulty by means of a photometer with divisions, which I had employed on another occasion. The result of this operation ensures a decided advantage to the new telescope. The pencil of rays reflected on the silvered glass is equal to 90 per cent. of those transmitted through an object-glass of four partial reflexions; so that the new instrument avails itself of the overplus of light, which, on account of the greater diameter of the mirror, concurs efficiently to the formation of the focal image. Diameters equal, the telescope with glass is by one-half shorter than the other instrument; with equal lengths, it bears a double diameter, and collects three and a half times more light. Considered in another point of view, the new combination is distinguished in this, that it produces all its effect without the concurrence of those numerous conditions required to obtain a certain degree of perfection in any telescope, whether reflecting or refracting. The achromatic telescope, above all, requires that the constructor of it, at one and the same time, pay particular attention to the homogeneity of the two sorts of glass which form the object-glass, their refracting and dispersive powers, the combination of curves, the centring and the execution of four spherical surfaces. In the new telescope, on the contrary, the glass, serving not as a middle refractor, but only to support a very thin layer of metal, the homogeneity of the mass is by no means required, and the most ordinary glass of sufficient thickness worked with care affords a concave surface, which when silvered and polished furnishes of itself and by reflexion excellent images. There is one strong objection to the metal mirrors,-it is, that they become oxidized in time, and are tarnished by contact with the air. Eight months I have kept silvered mirrors, which have not yet undergone any sensible alteration. Will they preserve this state of perfection a still longer time? The experiment has not beer sufficiently prolonged to decide one way or the other; but even should the lustre of the mirror become weaker, there is no difficulty in recurring to the same means for re-establishing it, by which it had been at first obtained. In fine, should the depth of the silver be altered, the operation of depositing it is so easy and prompt, that it can easily be repeated. To resume, the new instrument, compared with the refracting telescopes, gives, at much less cost, more light, more distinctness, and is free, like the reflecting telescope, from all aberration of refrangibility. On the Colour of Salts in Solution, each Constituent of which is coloured. By J. H. GLADSTONE, Ph.D., F.R.S. It is a general law, that "all the compounds of a particular base or acid, when in aqueous solution, absorb the same rays of light; "hence it may be deduced, that when a coloured base and a coloured acid combine, the resulting salt will transmit only those rays which are not absorbed by either constituent, or in other words, only those rays which are transmitted by both. This was proved to be actually the case by a prismatic examination of compounds of chromic, permanganic, and carbazotic acids with copper, iron, nickel, uranium, and chromium. Though the compounds of chlorine, bromine, and iodine with hydrogen and most metals are colourless, the compounds of these halogens with gold, platinum, and palladium exhibit an absorption of light due to the halogen as well as that due to the metal. The same is true in respect to chlorides, bromides, and iodides of copper, iron, nickel, and cobalt, when these salts are dissolved in a minimum of water; but when more water is added, the colour changes, and the absorption due to the halogen no longer exists. In one or two of the cases examined a slight variation from the general law occurred; and ferrocyanide of iron forms a complete exception. The double chloride of platinum and copper shows the absorbent effect of all three constituents. : On the Effects of Heat on the Colour of Dissolved Salts. If a coloured salt be dissolved in water, heating the solution does not usually affect the colour of it. In not a few cases, however, the colour is rendered more intense, and altered somewhat in its character. Among the examples mentioned were ferridcyanide of potassium, meconate of iron, chloride and bromide of palladium. In other cases, heating the solution produces apparently a total change of colour for instance, chloride of copper passes when heated from blue to green; chloride of nickel from a bluish to a yellowish green; sulphocyanide of cobalt, or chloride of cobalt dissolved in aqueous alcohol, from a pale red to a deep bluish purple. In all these instances heat causes the absorption of a larger quantity of rays by the solution; but this appears to depend sometimes upon some purely physical cause, at other times upon some chemical change. With ferridcyanide of potassium, and similar salts, a certain thickness of the heated solution produces precisely the same effect on the spectrum as an increased thickness of the same solution when cold. With chloride of copper, and similar salts, the somewhat dilute solution when heated produces the same effect on the spectrum as the same solution when concentrated and cold,-these salts being all of that character which is altered in colour by the addition of water. On Improvements in the Optical Details of Reflecting Telescopes and Equatoreal Instruments. By THOMAS GRUBE, M.R.I.A., Dublin. The author stated, that while the Earl of Rosse, by his achievements, had placed beyond doubt the practicability of producing specula for reflecting telescopes at once as perfect as could be desired, and as large as could be made practically useful, the achromatic object-glass had received but little increment of size; and though the Messrs. Chance, of Birmingham, had produced a pair of discs, of optical glass, of 29 inches diameter, yet these had been allowed to be transferred to another country, where the work of forming them into an object-glass was still to be effected. Four years had now elapsed since the production of these discs, and the refracting telescope may now be considered as being completely distanced in size by its competitor, the reflector. Under such circumstances, it was important, he conceived, to give to the reflecting telescope every possible accession of improvement which the progress of art or science placed from time to time at our disposal. The two points of admitted inferiority of the reflector being, a greater liability to tarnish than glass, and less intrinsic brilliancy of the reflected pencil of light; the author had succeeded (so far as the small speculum of the reflecting telescope is concerned) in entirely obviating the former objection, and in very much lessening the other. Regardless of the failure of an attempt, made years since, to construct a reflecting telescope of glass surfaces quicksilvered, he concluded, from his own experience, that such surfaces could be made equally perfect with those of speculum metal; while by silvering (not quicksilvering) that surface required to reflect, a great increase of light would result, thus producing for the small reflector of the telescope a mirror as imperishable as glass, and, in reflecting power, approaching the transmitting power of a lens. The author explained why, instead of using this reflector in its simplest form, viz. that of a lens of equal thickness silvered on one side, he preferred an achromatized compound of two lenses, cemented and silvered, and exhibited such a compound, which, he stated, had on trial performed perfectly. He next proceeded to describe in detail his proposed application of the same principles to both small and large specula of telescopes (where such were of moderate dimensions), as also an improved form of the prism of total reflexion applicable to Newtonian telescopes of the largest dimensions. This latter is a prism of divergent or concave power made aplanatic, or at least achromatized, reducing the convergence of the rays coming from the large speculum, and also the size required for the prism in the same arbitrary proportion (two or three times being suggested); the required magnifying power being obtained by a proportionally lower eye-piece. The author next proceeded to discuss the respective merits of the several varieties of equatoreal mounting as applicable to large telescopes. The first variety, or long polar axis (biforked or not), he rejects from its necessarily great length and consequent unsteadiness. The second, or large-cone polar axis, supporting the telescope in a bifurcation prolonged beyond the upper bearing, he would also reject, from the enormous weight of such in proportion to the telescope carried,-4 tons being stated as the moving mass in the case of a telescope of only 8 inches diameter. The third, or German variety of construction, the author considers, in its general type, as preferable to all others; and he has therefore devoted much attention to its improvement. By a system of internal counterpoise, he has reduced the direction of the pressure of the declination axis (with its appendages, including the telescope and its counterpoise) to that of the centre of revolution of the polar axis, removed all end pressure of the declination axis, and supported all but a small fraction of these weights by antifriction rollers. In this arrangement great steadiness is retained and freedom of motion attained. An instrument combining these principles and carrying a 12-inch achromatic of 20 feet focus, has but about 12 cwt. of material (including the telescope) to be moved; and this is effected by a force of about 1 lb. applied at the eye end. This instrument, contrasted with the 8-inch before mentioned, is (allowing for the difference in size) lighter in the proportion of about hundredweights to tons. The author, in conclusion, and aided by drawings, explained the general construction of an instrument of the German type which he had devised purposely for the proposed great southern telescope, and which construction had been selected by the Committee appointed by the British Association in reference to the same. In this instrument a telescope of the proposed diameter (viz. 4 feet), and the other moving portions of the instrument, are calculated at 19,000 lbs., moved by a force of 20 lbs., applied at a radius of 5 feet; the other proposed construction, which was that of the prolonged polar axis, being estimated at 45,000 lbs. moving weight, and requiring 750 lbs., or 37 times that of the author's construction for its movement. On a Method of determining whether the Luminiferous Vibrations are Parallel or Perpendicular to the Plane of Polarization. By M. L'ABBÉ MOIGNO, Paris. By a truly extraordinary tour de force, such as we find no other example of in the history of mathematics applied to physics, M. Cauchy, starting from two angles determined experimentally by Sir David Brewster, the principal angle of incidence and the |