Chap, xxvii.] ACHROMATIC LENS. 347 



of the two prisms. Thus it became possible to make 

 a convex lens of two different substances, so that 

 the dispersion produced by the first part of the lens 

 would be destroyed by the second, while the refrac- 

 tion, though diminished, still persisted. Rays of 

 light passing through such a lens are brought to a 

 focus without the accompaniment of rings of colour, 

 except to a very slight extent, the lens being prac- 

 tically achromatic. Such a combination is obtained 

 when crown glass and flint glass are used. A doubly 

 convex lens of crown glass is used, fitted to a concave 

 lens of flint glass. 

 The action of such 

 a lens is repre- 

 sented in Fig. 156, 

 where the ray P 



passing through Fig. 156. -A chroma tic Lens. 



the convex lens 



would be refracted, and at the same time dispersed, 

 so that the violet rays would be focussed at q, 

 and the red rays at q ; but the concave lens overcomes 

 the dispersion, diminishing at the same time the re- 

 fraction, and the ray P is focussed at Q'. 



Different substances do not disperse different 

 colours in the same ratio, so that while the total 

 dispersion by two substances may be made the same, 

 the dispersion of the colours between the extremes 

 may be in different proportions. A combination of 

 lenses, such as has been noted, will recombine two 

 given colours, but will not absolutely recombine the 

 others. The rays usually sought to be recombined 

 are the more luminous, such as orange and blue, and 

 this degree of achromatism is generally found sufficient, 

 though absolute achromatism can be obtained by 

 further combinations on the same principles. 



Spherical aberration. In speaking of mirrors 

 it was remarked (page 304) that it is not absolutely 



