ON OPTICAL INSTRUMENTS. 337 



focal length, this aberration would be only -^ as great ; it may, however, 

 1 be almost entirely corrected, in refracting telescopes, by employing proper 

 proportions in the dimensions of the various lenses. (Plate XXVIII. Fig. 

 412, 413.) 



A still more important aberration, from which reflecting telescopes are 

 also wholly free, is that which arises from the different refrangibilities of 

 the rays of light of different colours, which form an infinite number of 

 images, neither agreeing perfectly in situation nor in magnitude, so that 

 the objects are rendered indistinct by an appearance of colours at their 

 edgesrf this imperfection, however, Mr. Dollond has in great measure 

 obviated, by his achromatic object glasses : * the construction of which de- 

 pends on the important discovery, that some kinds of glass separate the 

 rays of different colours from each other much more than others, while the 

 whole deviation produced in the pencil of light is the same. Mr. Dollond 

 combined, therefore, a concave lens of flint glass with a convex lens of 

 crown glass, and sometimes with two such lenses ; the concave lens of flint 

 glass being sufficiently powerful to correct the whole dispersion of coloured 

 light produced by the crown glass, but not enough to destroy the effect of 

 its refraction, which was still sufficient to collect the rays of light into a 

 distant focus. For this purpose, it is necessary that the focal lengths of the 

 two lenses should be in the same proportion as the dispersive powers of the 

 respective substances, when the mean deviations of the pencils are equal ; 

 that is, in the case of the kinds of glass commonly used, nearly in the ratio 

 of 7 to 10. Sometimes also the chromatic aberration, that is, the error 

 arising from the different refrangibilities of the different rays, is partially 

 corrected in an eye piece, by placing a field glass in such a manner as con- 

 siderably to contract the dimensions of the image formed by the least 

 refrangible rays, which is nearest to the eye glass, and to cause it to subtend 

 an equal angle with the image formed by the most refrangible rays, 

 this image being little affected by the glass. (Plate XXVIII. Fig. 414, 

 415.) 



The apparent magnitude of an object viewed through a telescope, may 

 be measured, with great accuracy, by a scale or by wires, introduced at the 

 place of the last image, reducing afterwards the angle thus ascertained 

 according to the magnifying power. Care must, however, be taken to 

 avoid as much as possible the distortion which usually accompanies any 

 curvature of the image ; and the wires, one of which is sometimes made 

 moveable by means of a micrometer screw, must be sufficiently illuminated 

 to be distinctly visible. Sometimes a scale is introduced, which, from the 

 apparent magnitude of a known object, such as that of a man of ordinary 

 height, or of a portion of a wall built with bricks of the usual size, enables 

 us at once to read off its actual distance, which is expressed on the scale in 

 hundreds of yards. The angular magnitude of an object seen through a 

 telescope may also be found, by viewing at the same time, with the other 

 eye, either a scale, or any other object of known dimensions, placed at a 

 given distance : the lucid disc micrometer of Dr. Herschel t is employed 



* Ph. Tr. 1758, 1. 733, and 1765, Iv. 54. 

 f Ibid. 1782, Ixxii. 163; 1785, p. 46. 



