ON OPTICAL INSTRUMENTS. 431 



ance of a telescope, which are ahnost too intricate for practical calculation, 

 although some assistance may certainly be obtained fi'om theory with regard 

 to the most important of them. The curvature of the image produced by 

 any lens has already been mentioned: it may be in some measure remedied by 

 Mr. Ramsden's method of placing a planoconvex lens a little beyond the 

 image, with its, flat side turned towards it. !Mr. Ramsden also employs 

 an eye piece constructed on this principle instead of a simple microscope, 

 under the name of a double magnifier. The aberration of the different 

 parts of any single pencil of rays, from the corresponding point of the 

 image, requires also to be considered in the construction of telescopes: its 

 magnitude is such, in the case of a double convex lens of crown glass, 

 that those parts of a pencil of parallel rays which fall on it near the cir- 

 cumference meet each other in a point, which is within the true focus, by a 

 distance a little more than half as great again as the thickness of the lens. 

 In an image formed by a concave speculum, of equal focal length, this 

 aberration would be only i-V ^s great; it may, however, be slmost 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 edges: 

 this imperfection, however, Mr. DoUond has in great measure obviated, 

 by his achromatic object glasses: the construction of which depends on 

 the important discovery, that some kinds of glass separate the rays of differ- 

 ent 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 power- 

 ful 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 



