290 CORRECTION OF ABERRATIONS [CH. IX 



sion) differs very markedly in the two cases, although the total length 

 of the spectrum is the same in both. 



The red is much contracted and the blue expanded with the glass 

 prism. One can then have what might be called a mean refraction 

 with the glass prism, the refraction of the individual groups of wave 

 lengths not being in proportion to the lengths. Now it is from this 

 irregularity of the refraction in different parts of the spectrum, and 

 because the irregularity differs with different transparent substances, 



1 2 



FIG. 172. ACHROMATISM BY COMBINING DIFFERENT KINDS OF GLASS. 



(1) White light (W) traversing two equal crown glass (CC) prisms with their 

 bases opposite. The dispersion into a spectrum by the first prism is overcome by 

 the second prism and the light is recombined into a white beam (W 1 ), which is 

 displaced as if it had traversed a piece of plane glass. 



Red Blue The red and the blue edges of the spectrum. The blue is more 

 refracted than the red. 



(2) White light (W) traversing a flint glass prism (F) and being dispersed into 

 the spectral colors. The spectrum formed by the flint prism is recombined by the 

 crown glass prism (C), but the emerging ray of white light (W 2 ) is refracted mark- 

 edly toward the base of the crown glass prism, showing the possibility of an achro- 

 matic image. The arrows show the direction in which the light is extending. 



that it is possible to have the refraction necessary to produce images 

 without having the light dispersed into colors at the same time. This 

 is shown in fig. 172, 2, where a smaller prism of flint glass produces 

 the same amount of dispersion as a larger prism of crown glass. If 

 these prisms are with their edges opposite, the spectrum produced 

 by the flint glass will be brought together by the crown glass and 

 white light will result, but as the mean refraction of the larger crown 

 glass prism is greater than that of the flint glass prism, the ray of 

 white light will not extend parallel with the original direction, but 

 be bent toward the base of the crown glass prism. As a lens may be 

 considered an infinite number of prisms combined it becomes intelli- 



