90 Lord Rayleigh on Achromatic 



as regards the red and blue rays, the original bands may be 

 considered to be already obliterated, but so far as regards the 

 central rays, to be still fairly defined. Under these circum- 

 stances it is remarkable that so little colour should be apparent 

 on direct inspection of the bands. It would seem that the eye 

 is but little sensitive to colours thus presented, perhaps on 

 account of its own want of achromatism. 



It is interesting to observe the effect of coloured glasses 

 upon the distinctness of the bands. If the achromatism be in 

 the green, a red or orange glass, so far from acting as an aid 

 to distinctness, obliterates all the bands after the first few. On 

 the other hand, a green glass, absorbing rays for which the 

 bands are already confused, confers additional sharpness. With 

 the aid of a red glass a large number of bands are seen 

 distinctly, if the adjustment be made for this part of the 

 spectrum. 



A still better procedure is to isolate a limited part of the 

 spectrum by interposed screens. For this purpose a real 

 spectrum must be formed, as in the case of the grating above 

 considered. 



We will now inquire to what degree of approximation 

 X/b may be made independent of X with the aid of a prism, 

 taking Cauchy's law of dispersion as a basis. According to it 

 the value of b for any ray may be regarded as made up of two 

 parts — one constant, and one varying inversely as X 2 . We 

 therefore write 



*- X ' (12) 



where A is to be so chosen that X/b is stationary when X has 

 a prescribed value, X . This condition gives 



AV = 3B ; (13) 



so that 



h£=A ^ (U ) 



*A 3x Q x 2 -ix 2 ^ 



As an example, let us suppose that the disposition is achro- 

 matic for the immediate neighbourhood of the line D, so that 

 \ = X D , and inquire into the proportional variation of X/b, 

 when we consider the ray C. Assuming 



X D = -58890, X c = -65618, 

 we obtain from (14) 



V*= 1-0155. 



Xq/Oq 



The meaning of this result will be best understood if we 

 inquire for what order (n) the bands of the C-system are 



