OPTICS. 



27 



Isaac would have been correct in con- 

 cluding that " the improvement of the 

 refracting telescope teas desperate" 

 (Optics, Part I. Prop. VII. Theor. VI.) 



Sir Isaac, however, was in this case 

 mistaken, and it was reserved for our 

 countryman, John Dollond, to prove, by 

 direct experiment, that refraction can 

 be produced free of colour, and actually 

 to construct an achromatic* telescope, 

 or a telescope without colour. 



We have already shown that all 

 bodies have different dispersive powers ; 

 and it will be seen from our Table, that 

 flint glass (the white glass of which 

 drinking glasses are made), and crown 

 glass (the glass with which windows are 

 glazed), have different powers of pro- 

 ducing colour. 



If we now make the prism ABC, 

 Jig. 29, of crown glass, and A b C of 

 flint glass, and make the angle b A C 

 such that it corrects the colour of A C B, 

 or produces a spectrum of exactly the 

 same length, then since the flint glass 

 does this at a much less mean refrac- 

 tion than the crown glass, it will refract 

 the rays from the prism A C B to a 

 point Y', above Y, where they will be 

 all collected into a circle of white light. 

 We have, therefore, succeeded in re- 

 fracting the beam S F Y into a new 

 direction, S F Y, without colour. 



The application of this to lenses is 

 shown in Jig. 3 1 , which is drawn so as 

 to admit of comparison with/#. 29. In 

 this figure, L L is acorns lens of crown 

 glass, and / / a concave one of flint glass. 



A ray of the sun, S, falls at F on the 

 convex lens, which will refract it exactly 

 in the same manner as the prism A B C, 

 whose faces touch the two surfaces of 

 the lens at the points where the ray 

 enters and quits it. The solar ray, S F, 

 thus refracted by the lens L L, or prism 

 ABC, would have formed a spectrum 

 P T on the wall, had there been no other 

 lens, the violet ray F V crossing the 

 axis of the lens at V, and going to the 

 upper end P of the spectrum, and the 

 red ray F R going to the lower end T. 

 But as the flint glass lens / / or the 

 prism A C which receives the rays F V, 

 F R at the same points, is interposed, 

 these rays will be united at/, and form 

 a small circle of white light, the ray 

 S F of the sun being now refracted 

 without colour from its primitive direc- 

 tion SFYinto the new direction F/. 

 In like manner the corresponding ray 

 S' F' will be refracted to/, and a white 

 and colourless image of the sun will be 

 there formed by the two lenses. 



In this combination of lenses, it is 



* From two Greek words which signify without wlwr. 



evident, that the spherical aberration 

 of the flint lens corrects to a consider- 

 able degree that of the crown one, and, 

 by a proper adjustment of the radii of 

 the surfaces, it may be almost wholly 

 removed. This, however, is more per- 

 fectly effected in the triple achromatic 

 object-glass, which consists of three 

 lenses, viz. a concave flint glass lens 

 placed between two convex lenses of 

 crown glass ; but this form of the achro- 

 matic object-glass is now generally 

 abandoned, and almost all the large 

 object-glasses which have been recently 

 constructed consist of two lenses only. 

 In the treatise on optical instruments, 

 the reader will find the practical details 

 respecting achromatic object-glasses and 

 eye-glasses. 



When we examine with attention the 

 best achromatic telescope, we shall find 

 that it does not show white or luminous 

 objects perfectly free from colour, then- 

 edges being tinged on one side with a 

 claret-coloured fringe, and, on the other, 

 with a green fringe. These unconnected 

 colours, which have been called the 



