GALILEAN TELESCOPE.] 



UNDULATORY FORCES. LIGHT. 



73 



optical effects produced in the employment of telescopes 

 (See Fig. 57). The larger lens at L being the object- 

 glass, receives the rays of light, which, for our pi\ 

 purpose, we shall suppose to proceed from a star, A B ; 

 and are therefore, practically speaking, parallel to each 

 other. The image of these rays is found at a 6, and is, 

 of course, in veiled. The lens, or eye-piece, L', magnifies 

 the image, a 6, in the manner represented at 6' a'. 

 Fig. 57. 



In all telescopes, arrangements are made by means of 

 Inch the relative distances between the object-glass and 

 the eye-piece can be varied. This is for the puipose of 

 adapting the instrument to the optical peculiarities of 

 each observer ; and is necessary, because the point of 

 distinct vision varies considerably in different individuals, 

 owing to the convexity of the eye in some, and its com- 

 parative concavity in others. We have already spoken 

 fully of this subject in our remarks on the human eye ;* 

 to which we refer those of our readers who may be 

 desirous of observing their application in reference to 

 use of telescopes, and other instruments generally. 

 If two eye-pieces, of equal focal length, are placed suc- 

 ennively between the object-glass and the eye-piece of 

 .-lescope represented iu Fig. 57, the image will be 

 presented to the eye iu a non inverted form. This is, in 

 fact, the arrangement employed in all ordinary telescopes 

 used for viewing terrestrial objects ; and the magnifying 

 power of such instruments, as in the one we have been 

 considering, is found by dividing the focal length of the 

 object glass by that of one of the eye-glasses. The 

 non-inversion, however, of the image may be obtained 

 using a double concave eye-piece, instead of a com . x 

 one ; the lens being placed nearer to the object-glass 

 than at the point at winch the rays passing therefrom 

 would form ill image. Fig. 58 represents an arrange- 

 f\f. 48. nii-iit of this 



kind, iu which 

 a 6 is the distant 

 object ; /, tho ob- 

 ject-glass ; f , the 

 concave lens ; 

 6 , the place at 

 which the in- 

 verted image 

 produced from the object-glass would be formed; and 

 j a' 6' is the non-inverted imago, as observed by the eye of 

 the spectator. The length of the instrument is much 

 diminished by this arrangement ; and hence it is a con- 

 venient form for opera-glasses, and other similar instru- 

 ments, in which a moderate magnifying power, and 

 limited field of view, only are required. In this instru- 

 ment the two lenses are placed at a point equal to the 

 difference of their focal lengths ; whilst, when two convex 

 glasses are employed, they are separated to a distance 

 1 to the sum of their focal lengths. This instrument 

 i Tally called the Galilean telescope, from its having 

 employed by Galileo, the celebrated Italian phi- 

 n;r. 



Having thus described tho ordinary forms of the tele- 

 scope, we proceed to those of a higher class, in which 

 chromatic aberration is almost wholly destroyed by the 

 use of what arc termed achromatic lenses. We have 

 already explained the principles on which they are made, 

 when treating on dispersion and achromatism ;t it there- 

 fore only remains for us to describe the advantages 

 derived from their adoption in telescopes. Mathemati- 

 xpeaking, two lenses may be so arranged in ordi- 

 nary telescopes, that one, being the object-glass, shall 

 liave a focal distance of any assignable length, whilst 

 the ether, or eye-piece, may equally have one of the 



x ante, p. 48. f Ant,, p. 53, 



VOL, L 



smallest extent ; and the magnifying power of a telescope 

 constructed of two such lenses, will, as we have already 

 explained, be just in the ratio of the large focal distance 

 of the object-glass divided by the short one of the eye- 

 piece. But practically, there are limits, both of con- 

 struction and use, which forbid the employment of 

 lenses beyond a certain size and focal length. In the 

 earlier days of telescopic discovery, object-glasses were 

 used which had focal lengths varying from three to three 

 hundred feet. Of these it will be unnecessary to 

 do more than state, that their use was highly in- 

 convenient. But to employ lenses of ordinary 



^ glass for high magnifying power, and at but a 



B" manageable distance from each other, was an un- 

 solved problem until Mr. Dollond discovered, that 

 a combination of lenses of two kinds of glass, 

 whose dispersive and refractive powers would 

 correct each other, effected that purpose. An object- 

 glass composed of a concave lens of Iliut glass, into 

 which is fixed a double convex lens of crown glass, 

 meets the difficulty ; and telescopes fitted with such, 

 give not only a clear and distinct image, owing to an 

 almost entire freedom from both chromatic and spherical 

 aberration, but also supply another great boon to the 

 astronomer, arising from this effect namely, that of 

 permitting the use of a telescope not exceeding a few 

 feet in length, in place of those of the extraordinary size, 

 to which we have just alluded. In such arrangements, 

 magnifying powers of astonishing extent may be em- 

 ployed ; and thus the astronomer is supplied with 

 an instrument whose capabilities are, practically, only 

 limited by the means of its owner, and the skill of the 

 workman employed in its construction. 



REFLECTING TELESCOPES. 



\\ K now proceed to examine another kind of telescope, 

 whose use is almost entirely confined to astronomical 

 purposes, and in the construction of which great in- 

 genuity has been manifested. 



A reflecting telescope consists chiefly of three parts 

 namely, the speculum, or reflector, which receives the 

 rays from the object (this takes the place of the object- 

 glass in the refracting telescope) ; the smaller reflector, 

 which is sometimes employed to receive the imago from 

 the large one ; and the eye-piece, by means of wlu'ch the 

 image is viewed and magnified. These arrangements 

 either vary or are modified iu different telescopes, as we 

 shall presently see. 



It will be unnecessary that we should go over tho 

 same ground again in describing tho general character, 

 etc., of vaiious forms of curved reflectors. This we have 

 already done in a former part of this work, somewhat 

 minutely ; and we therefore refer our readers thereto,! 

 and will proceed to describe each of the parts of the 

 telescope in detail. 



The speculum is generally made of an alloy composed 

 of variable quantities of copper and tin ; in addition to 

 which, small proportions of other metals have been used. 

 It is impossible to give the exact proportions in which 

 these metals should be employed to obtain the ljc:.t 

 result. The object sought after is an alloy wlu'ch shall 

 be able to receive the highest polish ; to retain this as 

 long as possible ; and yet, despite the hardness necessary 

 to these ends, it must be sufficiently yielding to undergo 

 the process of grinding. After being cast in a mould of 

 suitable lorm, the alloy is annealed, and its brittle nature 

 is thus tempered down. Tho process of grinding tho 

 casting into a proper shape then follows, and is effected 

 by pressing on its surface a wooden mould which has 

 been turned to a shape convex to the concave one which 

 the speculum has to receive. Whilst the speculum is 

 made to revolve, tho mould, or grinder, is continually 

 being covered with the powder employed in grinding. 

 In some cases the speculum is stationary whilst the 

 polishing mould or tool rotates against it. This opera- 

 tion is one of great delicacy, as on tho perfection of 

 shape and polish, the value of the speculum entirely 

 j See ante, p. 43. 



