Abbildungen der Seite
PDF
EPUB

ad we perceive only the image of that image formed at z. Nevertheless, fuch telescopes are exceedingly diftinct, and reprefent objects fo clearly as to be preferred, in viewing_terrestrial things, even to reflectors themselves. The latter indeed have greatly the advantage in their powers of magnifying, but they are very deficient in point of light. Much more light is loft by reflection than by refraction: and as in thefe telescopes the light muft unavoidably fuffer two reflections, a great deal of it is loft; nor is this lofs counterbalanced by the greater aperture which these telefcopes will bear, which enables them to receive a greater quantity of light than the refracting ones. The metals of reflecting telescopes alfo are very much fubject to tarnish, and require much more dexterity to clean them than the glaffes of refractors; which makes them more troublesome and expenfive, though for making discoveries in the celeftial regions they are undoubtedly the only proper inftruments which have been hitherto conftructed. If Dr BLAIR indeed shall discover a vitreous fubftance of the fame powers with the fluid in the compound object-glafs of his telescope, (and from his abilities and perfeverance we have every thing to hope,) a refracting telescope may be constructed fuperior for every purpose to the beft reflector.

592. II. THE REFLECTING TELESCOPE. The inconveniencies arifing from the great length of refracting telescopes, before Dollond's difcovery, are obvious; and thefe, together with the difficulties occafioned by the different refrangibility of light, induced Sir Ifaac Newton to turn his attention to the fubject of reflection, and endeavour to realize the ideas of himself and others, concerning the poffibility of conftructing telescopes upon that principle. The inftrument which he contrived is reprefented, (fig. 2. pl. 261.) where ABCD is a large tube, open at AD and closed at BC, and of a length at least equal to the distance of the focus from the metallic fpherical concave fpeculum GH placed at the end BC. The rays EG, FH, &c. proceeding from a remote object PR, interfect one another fomewhere before they enter the tube, so that EG, eg, are thofe that come from the lower part of the object, and fh, FH from its upper part: these rays, after falling on the fpeculum GH, will be reflected, so as to converge and meet in mn, where they will form a perfect image of the object.-But as this image cannot be feen by the fpectator, they are intercepted by a fmall plane metallic fpeculum KK, interfecting the axis at an angle of 45°, by which the rays tending to mn will be reflected towards a hole LL in the fide of the tube, and the image of the object will thus be formed in q S; which image will be lefs diftinct, because some of the rays which would otherwise fall on the concave fpeculum GH, are intercepted by the plane fpeculum: nevertheless it will appear in a confiderable degree diftinct, because the aperture AD of the tube, and the fpeculum GH are large. In the lateral hole LL is fixed a convex lens, whofe focus is at Sq; and therefore this lens will refract the rays that proceed from any point of the image, fo as at their exit they will be parallel, and thofe that proceed from the extreme points S q will converge after

refraction, and form an angle at O, where the eye is placed; which will fee the image Sq, as if it were an object, through the lens LL; confequently the object will appear enlarged, inverted, bright, and diftin&t. In LL lenses of different convexities may be placed, which by being moved nearer to the image, or farther from it, would represent the object more or lefs magnified, provided that the furface of the fpeculum GH be of a perfectly fpherical figure. If, in the room of one lens LL, 3 lenfes be difpofed in the fame manner with the three eye-glaffes of the refracting telescope, the object will appear erect, but lefs diftin&t than when it is obferved with one lens. On account of the pofition of the eye in this telescope, it is extremely difficult to direct the inftrument towards any object. HUYGENS, therefore, first thought of adding to it a small refracting telescope, the axis of which is parallel to that of the reflector. This is called a finder, or director. The Newtonian telescope is alfo furnished with a fuitable apparatus for the commodious use of it.

593. To determine the magnifying power of this telescope, it is to be confidered that the plane fpeculum KK is of no ufe in this refpect. Let us then fuppofe, that one ray proceeding from the object coincides with the axis GLIA (fig. 8. Pl. 259.) of the lens and fpeculum; let bb be ano ther ray proceeding from the lower extreme of the object, and paffing through the focus I of the fpeculum KH: this will be reflected in the direction bid, parallel to the axis GLA, and falling on the lens dLd, will be refracted to G; so that GL will be equal to LI, and dG=¿I. To the naked eye the object would appear under the angle 16ibIA; but by means of the telescope it appears under the angle d GLd IL=Idi; and the angle Idi is to the angle Ibi::b1:1d; confequently the apparent magnitude by the telefcope is to that by the naked eye, as the diftance of the focus of the speculum from the fpeculum, to the diftance of the focus of the lens from the lens.

594. The Newtonian telescope was ftill inconvenient. Notwithstanding the contrivance of HUYGENS, objects were by it found with difficulty. The GREGORIAN TELESCOPE, therefore, foon obtained the preference, to which for moft purposes it is juftly entitled, as is evident from the following conftruction: Let TYYT (fig. 3. Pl. 261.) be a brass tube, in which LldD is a metallic concave fpeculum, perforated in the middle at X; and EF à lefs concave mirror, so fixed by the arm or ftrong wire RT, which is moveable by means of a fcrew on the outfide of the tube, as to be moved nearer to, or farther from, the larger fpeculum LldD, its axis being kept in the fame line with that of the great one. Let AB reprefent a very remote object, from each part of which iffue pencils of rays, fuch as e d, CD, from A the upper extreme of the object, and IL, il, from the lower part B; the rays IL, CD from the extremes croffing one another before they enter the tube. Thefe rays, falling upon the larger mirror LD, are reflected from it into the focus KH, where they form an inverted image of the object AB, as in the Newtonian telescope. From this image the rays, iffuing as from an object, fall

upou

upon the small mirror EF, the centre of which is at e; fo that after reflection they would meet in their foci at QQ, and there form an erect image. But fince an eye at that place could fee but a fmall part of an object, in order to bring rays from more diftant parts of it into the pupil, they are intercepted by the plano-convex lens MN, by which means a fmaller erect image is formed at PV, which is viewed through the menifcus SS by an eye at O. This menifcus both makes the rays of each pencil parallel, and magnifies the image PV. At this place of the image all the foreign rays are intercepted by the perforated partition ZZ. For the fame reafon the hole near the eye Ọ is very narrow. When nearer objects are viewed by this telescope, the fmall fpeculum EF is removed to a greater diftance from the larger LD, fo that the 2d image may be always formed in PV; and this diftance is to be adjusted (by the fcrew on the outfide of the great tube) ac cording to the form of the eye of the fpectator. It is alfo neceffary, that the axis of the telescope fhould pass through the middle of the fpeculum LL, and the middle of the hole X, the centres of the lenfes MN, SS, and the hole near O. As the hole X in the fpeculum LL can reflect none of the rays iffuing from the object, that part of the image which correfponds to the middle of the object must appear to the obferver more dark and confused than the extreme parts of it. Befides, the speculum EF will alfo intercept many rays proceeding from the object; and therefore, unlefs the aperture TT be large, the object muft appear in fome degree obfcure.

595. In the beft reflecting telescopes, the focus of the fmall mirror is never coincident with the focus of the great one, where the firft image KH is formed, but a little beyond it (with refpect to the eye), as at n; the confequence of which is, that the rays of the pencils will not be parallel after reflection from the fmall mirror, but converge fo as to meet in points about Q9Q, where they would form a larger upright image than PV, if the glafs R was not in their way; and this image might be viewed by a fingle eye-glafs properly placed between the image and the eye; but then the field of view would be lefs, and confequently not so pleasant; for which reafon, the glafs R is ftill retained, to enlarge the fcope or area of the field. To find the magnifying power of this telefcope, multiply the focal diftance of the great mirror by the diftance of the fmall mirror from the image next the eye, and multiply the focal diftance of the fmall mirror by the focal diftance of the eye-glafs: then divide the product of the former multiplication by the product of the latter, and the quotient will exprefs the magnifying power.

596. One great advantage of the reflecting telefcope is, that it will admit of an eye-glafs of a much shorter focal distance than a refracting telefcope will; and confequently it will magnify fo much the more; for the rays are not coloured by reflection from a concave mirror, if it be ground to a true figure, as they are by paffing through a convex gl.., let it be ground ever fo true. The nearer an object is to the telescope, the more its pencils of rays will diverge before they fall upon

VOL. XVI. PART II.

the great mirror, and therefore they will be the longer of meeting in points after reflection; fo that the firft image KH will be formed at a greater distance from the large mirror, when the object is near the telescope, than when it is very remote. But as this image must be formed farther from the fmall mirror than its principal focus 7, this mirror must be always fet at a greater diftance from the large one, in viewing near objects, than in viewing remote ones. And this is done by turning the fcrew on the outfide of the tube, until the fmall mirror be so adjusted, that the object (or rather its image) appears perfect.

[ocr errors]

597. In looking through any telescope towards an object, we never fee the object itself, but only that image of it which is formed next the eye in the telescope. For if a man holds his finger or a ftick between his bare eye and an object, it will hide part (if not the whole) of the object from his view: But if he ties a ftick across the mouth of a telescope before the object-glafs, it will hide no part of the imaginary object he saw through the telescope before, unless it covers the whole mouth of the tube: for all the effect will be, to make the object appear dimmer, because it intercepts part of the rays. Whereas, if he puts only a piece of wire across the infide of the tube, between the eye-glafs and his eye, it will hide part of the object which he thinks he fees; which proves, that he fees not the real object, but its image. This is alfo confirmed by means of the fmall mirror EF, (fig. 3. Pl. 261.) in the reflecting telescope, which is made of opaque metal, and stands directly be tween the eye and the object towards which the telefcope is turned; and will hide the whole object from the eye at O, if the two glasses ZZ and SS are taken out of the tube.

598. Great improvements have been lately made in the construction of both reflecting and refracting telescopes, as well as in the method of apply. ing thofe inftruments to the purposes for which they are intended. Thefe, however, fall not properly under the science of optics, as fitter opportunities occur of giving a full account of them, as well as of the magic lantern, camera obfcura, &c. under the other articles of our multifarious work. See CATOPTRICS, DIOPTRICS, SPECULUM, and TELESCOPE.

SV. Ofthe different MERITS of MICROSCOPES and TELESCOPES; the DISCOVERIES made by them, and of further IMPROVEMENTS.

599. THE advantages arising from the ufe of microfcopes and telescopes depend, in the first place, upon their property of magnifying the minute parts of objects, fo that they can be thus more diftinctly viewed by the eye; and 2dly, upon their throwing more light into the pupil of the eye than what is done without them. The advantages arifing from the magnifying power would be extremely limited, if they were not alfo accompanied by the latter: for if the fame quantity of light is fpread over a large portion of furface, it becomes proportionably diminished in force; and therefore the objects, though magnified, appear proportionably dim. Thus, though any magnifying glafs fhould enlarge the diameter of the object 10 times, and confequently magnify the furface 100 times, yet if the focal distance of

Iii

the

the glass was above 8 inches (provided this was poffible), and its diameter only about the fize of the pupil of the eye, the object would appear 100 times more dim when we looked through the glafs, than when we beheld it with our naked eyes; and this, even on a fuppofition that the glafs tranfmitted all the light which fell upon it, which no glafs can do. But if the focal diftance of the glafs was only 4 inches, though its diameter remained as before, the inconvenience would be vaftly diminished, because the glafs could then be placed twice as near the object as before, and confequently would receive 4 times as many rays as in the former cafe, and therefore we would fee it much brighter than before. Going on thus, ftill diminishing the focal distance of the glafs, and keeping its diameter as large as poffible, we will perceive the object more and more magnified, and at the fame time very diftin&t and bright. It is evident, however, that with regard to optical inftruments of the microfcopic kind, we muft fooner or later arrive at a limit which cannot be paffed. This limit is formed by the following particulars: 1. The quantity of light loft in pafsing through the glafs. 2. The diminution of the glafs itfelf, by which it receives only a fmall quantity of rays. 3. The extreme fhortnefs of the focal diftance of great magnifiers, whereby the free accefs of the light to the object which we wish to view is impeded, and confequently the reflection of the light from it is weakened. 4. The aberrations of the rays, occafioned by their different refrangibility.

600. To understand this more fully, as well as to see how far these obstacles can be removed, let us fuppofe the lens made of fuch a dull kind of glafs, that it tranfmits only one half of the light which falls upon it. It is evident that fuch a glafs, of 4 inches focal distance, and which magnifies the diameter of an object twice, ftill fuppofing its own breadth equal to that of the pupil of the eye, will how it 4 times magnified in the furface, but only half as bright as if it was feen by the naked eye at the ufual distance; for the light which falls upon the eye from the object at 8 inches diftance, and likewife the furface of the object in its natural fize, being both represented by 1, the furface of the magnified object will be 4, and the light which makes that magnified object visible only 2; because though the glafs receives 4 times as much light as the naked eye does at the ufual diftance of diftinct vifion, yet one half is loft in paffing through the glafs. The inconvenience in this refpect can therefore be removed only as far as it is poffible to increase the clearness of the glafs, fo that it fhall tranfmit nearly all the rays which fall upon it; and how far this can be done, hath not yet been af certained...

601. The ad obftacle to the perfection of microscopic glasses is the small size of great magnifiers, by which, notwithstanding their near approach to the object, they receive a smaller quantity of rays than might by expected. Thus, fuppofe a glafs of only one 10th of an inch focal dif. tance; fuch a glafs would increase the vifible diameter 80 times, and the furface 6400 times. If the breadth of the glass could at the fame time be

preferved as great as that of the pupil of the eye, which we fhall fuppofe two roths of an inch, the object would appear magnified 6400 times, at the faine time that every part of it would be as bright as it appears to the naked eye. But if we fuppofe that this magnifying glafs is only one 26th of an inch in diameter, it will then only receive 4th of the light which otherwife would have falen upon it; and therefore, inftead of communicating to the magnified object a quantity of illu mination equal to 6400, it would communicate only one equal to 1600, and the magnified object would appear 4 times as dim as it does to the naked eye. This inconvenience, however, is ftill capable of being removed, not indeed by increafing the diameter of the lens, because this must be in proportion to its focal diftance, but by throwing a greater quantity of light on the ob ject. Thus, in the above-mentioned example, if 4 times the quantity of light which naturally falls upon it could be thrown upon the object, it is plain that the reflection from it would be 4 times as great as in the natural way; and confequently the magnified image, at the fame time that it was as many times magnified as before, would be as bright as when feen by the naked eye. In tranfparent objects this can be done very_effectually by a concave ipeculum, as in the reflecting microscope already defcribed: but in opaque objects the cafe is fomewhat doubtful; neither do the contrivances for viewing thefe objects feem entirely to make up for the deficiencies of the light from the fmallness of the lens and shortness of the focus.-When a microfcopic lens magnifies the diameter of an object 40 times, it hath then the utmoft poffible magnifying power, without dimi. nifhing the natural brightness of the object.

602. The 3d obftacle arifes from the shortnefs of the focal diftance in large magnifiers; but in transparent objects, where a fufficient quantity of light is thrown on the object from below, the inconvenience arifes at laft from ftraining the eye, which must be placed nearer the glass than it can well bear; and this entirely fuperfedes the ufe of magnifiers beyond a certain degree.

603. The 4th obftacle arifes from the different refrangibility of the rays of light, and which frequently caufes fuch a deviation from truth in the appearance of things, that many people have ima gined themselves to have made furprising difcoveries and have even published them to the world; when in fact they have been only as many optical deceptions, owing to the unequal refractions of the rays. For this there feems to be no remedy, except the introduction of achromatic glaffes into microscopes as well as telescopes. How far this is practicable, hath not yet been tried; but when thefe glaffes fhall be introduced (if practicable), microfcopes will then undoubtedly have received their ultimate degree of perfection.

604. With regard to telescopes, thofe of the refracting kind have evidently the advantage of all others, where the aperture is equal, and the aberrations of the rays are corrected according to Mr Dollond's method; because the image is not only more perfect, but a much greater quantity of light is tranfmitted than can be reflected from the best materials hitherto known. Unluckily, however,

the

the imperfections of the glass fet a limit to thefe telescopes, fo that they cannot be made above 3 feet long. On the whole, therefore, the reflecting telescopes are preferable in this refpect, that they may be made of dimenfions greatly fuperior; by which means they can both magnify to a great er degree, and at the same time throw much more light into the eye.

605. With regard to the powers of telescopes, however, they are all of them exceedingly lefs than we would be apt to imagine, from the num-, ber of times which they magnify the object. Thus, when we hear of a telescope which magnifies 200 times, we are apt to imagine, that, on looking at any diftant object through it, we should perceive it as diftinctly as we would with our naked eye at the 200th part of the diftance. But this is by no means the cafe; neither is there any theory capable of directing us in this matter: we muft therefore depend entirely on experience.

606. The best method of trying the goodness of any telescope is by obferving, how much farther off you are able to read with it than you can with the naked eye. But that all deception may be avoided, it is proper to choose fomething to be read where the imagination cannot give any affif. tance, such as a table of logarithms, or fomething which confifts entirely of figures; and hence the truly useful power of the telescope is eafily known. In this way Mr Short's large telescope, which magnifies the diameter of objects 1200 times, is yet unable to afford sufficient light for reading at more than 200 times the distance at which we can read with our naked eye.

607. With regard to the form of reflecting telescopes, it is now pretty generally agreed, that when the Gregorian ones are well conftructed, they have the advantage of thofe of the Newtonian form. One evident advantage is, that with the Gregorian telescope an object is perceived by looking directly through it, and confequently is found with much greater eafe than in the Newtonian telescope, where we muft look into the fide. The unavoidable imperfection of the fpecu. la, common to both, alfo gives the Gregorian an advantage over the Newtonian form. Notwithftanding the utmost care and labour of the workmen, it is found impoffible to give the metals either a perfectly spherical or a perfectly paraboli

A.

ABERRATION difcovered, 176;
defined, 274, 536. theory of it,
535-552. evils arifing from it,
536, $31. remedy, 338-552.
Abforption of rays of light, dif-
covered, 102.
Achromatic telescope, 44.
Adams, Mr G. improves the mi-
crofcope, 207, 212. his globu.
lar magnifiers, 207.
Epinus, M. improves the tele-
fcope, 199. obferves an opti-
cal phenomenon, 343-

cal form. Hence arifes fome indiftinctness of the image formed by the great speculum, which is frequently corrected by the little one, provided they are properly matched. But if this is not done the error will be made much worfe; and hence many of the Gregorian telescopes are far inferior to the Newtonian ones; namely when the specula have not been properly adapted to each other. There is no method by which the workman can know the fpecula which will fit one another without a trial; and therefore there is a neceffity for having many specula ready made of each fort, that in fitting up a telescope those may be chofen which beft fuit each other.

608. The brightness of any object seen through a telescope, in comparison with its brightness when feen by the naked eye, may in all cafes be eafily found by the following formula: Let n represent the natural diftance of a vifible object, at which it can be distinctly feen; and let dreprefent its diftance from the object-glafs of the inftrument. Let m be the magnifying power of the inftrument; that is, let the visual angle fubtended at the eye by the object when at the diftance n, and viewed without the inftrument, be to the visual angle produced by the inftrument as I to m. Let a be the diameter of the object-glafs, and p be that of the pupil. Let the inftrument be so constructed, that no parts of the pencils are intercepted for want of fufficient apertures of the intermediate glaffes. Laftly, let the light lost in reflection or refraction be neglected.

an

609. The brightness of vision through the in-
ftrument will be expressed by the fraction
the brightness of natural vifion being 1. But al
mpd
though this fraction may exceed unity, the vifion
through the inftrument will not be brighter than
natural vifion. For, when this is the cafe, the
pupil does not receive all the light transmitted
through the inftrument.

diftinct vifion, nearly 8 inches. But a difference
610. In microscopes, n is the nearest limit of
in this circumftance, arifing from a difference in
the eye, makes no change in the formula, because
m changes in the fame proportion with n.
611. In telescopes, n and d may be accounted
equal, and the formula becomes-

[blocks in formation]

'mp1

[blocks in formation]
[blocks in formation]

Bacon, lord Verulam, made few

discoveries in optics, 70. Bacon, Roger, his optical discoveries, 11, 163. his mistakes, 13. is faid to have apparently walked in the air, 70. Baker, Mr, his account of microfcopes, 205, 213, 576, 577Barker, Dr, his reflecting microscope, 209. Barrow, Dr, his optical difficulty, 355. his theory of place, diftance, &c. 473, 474Beams, diverging, phenomena of, 514-517.

[ocr errors]

Beaume, M. affifts M. Nollet in his experiments, 116. Beguelin, M. his discoveries, 41,

45, 258. Berkeley, Bp. his theory of vifion, 161, 481. Bernoulli, Dan. his experiment

and calculation, 324. Binocular telescope, 177. origin of it, and phenomena fhown by it, 342. Black marble reflects powerfully, 37

Blair, Dr, his difcoveries, 56. he improves the telescope, ib. 588. propofes a form of obfervation of refraction, 253. Bodies, weight neceffary to bring into contact, 155. Borelli, or quoted, 166, 200. he Borellus, improves telescopes,

179.

Bofcovich, F. his discoveries, 55. proposes an experiment, 247. detects a miftake of Sir I. Newton, 546.

Bouguer, M. his experiments on the quantity of light loft by reflection, 87-91, 93-111. bis difcoveries, 97-101. his hy. pothefis as to the apparent distance, place, &c. of objects, 483, 493-498. his reafon for the blue colour of the fky, 508. his hypothefis of the blue and green fhadows, 510-512. his methods of measuring light, 525-532.

Boyle, R. his difcoveries, 18, 75.

and experiments, 75-82. Brahe, Taycho, his optical difcoveries, 9.

Brereton, Lord, his exhibition of glass coloured by the air, 85.

Briggs, Dr, his hypothefis offingle

vilion, 235. Brilliancy, phenomena of, 249

251, 257. Briftle, experiment with a, 143. Buffon, Count, his experiments on the diminution of light, 92. his account of the blue and green fhadows, 511. Burning-glaffes, known to the ancients, 68, 69. large one made by Count Buffon, 92. Abbe Nollet's experiments with, 115, 116.

C.

Caille, M. De La, publishes M.

Bouguer's works, 87.

Camera Obfcura discovered, 15. Campani, M. improves telescopes, 178.

Candle, light of a, exhibits the

fame phenomena with that of the fun, 255. compared with day light, 334. Caffegrain, his vain attempt, 188, Caffini, M. his discoveries, 22, 178.

Cat, experiment with a, 320. Cat, M. Le, his difcoveries and experiments, 157. his hypothefis of vifion, 321, 476. his account of an optical deception,

503.

Catacauftics defined, 544. Catoptrics defined, 216.

Cavallieri, M. his discoveries, 173.

Caustics, optical, 544. Chefelden, Dr, cafe related by 335.

Cheft, Mr, his discovery, 54. Choroides defcribed, 308. fup

posed to be the feat of vifion, 315, 316. difputed, 317–327. Chromatics defined, 216. Cimento, academy of the, their fruitless experiment, 116. Circles of diffusion, 546. Clairaut, M. his opinion of Euler's hypothefis, 33. his theory, of refrangibility, and experi ments on it, 41-43. Clichtovæus, his hypothefis of the rainbow, 425. Cold, reafon why it is moft intenfe on the tops of high mountains, 114. Colours, difcoveries respecting,

27-29. experiments, 75-85. Newton's theory of, 415. Comets Euler's hypothefis re fpecting, 254. Compound light, 415. Concave glaffes made, 165. phenomena of objects seen through them, 357. concave furfaces, laws of reflection from, 370372. proved, 377-382. concave mirrors, 558–562. Concavity of the sky, 505. Concavo-convex glass, described,

291.

Contact between bodies, not proved, 155.

Convex glaffes, discovery of, 163. phenomena of objects feen through them, 354-356.table of their magnifying powers, 574. convex mirrors, 558562. convex furfaces, laws of reflection from, 373-376.proved, 377-382. how to find the focal diftance of rays reflected from, 395, 396. Cornea described, 308. Coronas. See Haloes. Coline of retraction, an improve

ment, refpecting it, 541. Courtivron, M. his opinion of the leaft angle of divifion,

[blocks in formation]
« ZurückWeiter »