WOOD. — NITROSO-DIMETHYL-ANILINE. 63 



For a medium having but a single absorption band the formula takes 



the form 



2 - m'A 2 



n 2 = 1 + 



A 2 - A' 2 



•This formula can, I think, be applied to sodium vapor, judging from 

 results recently obtained. Most other substances have, however, a 

 second band further along in the ultra-violet, though for the region 

 investigated experimentally this is so far removed that m" can be con- 

 sidered constant. The formula then takes the form 



2 , m ' x2 



n* — m + r-f 



A 2 - A' 2 ' 



in which m = m" + 1. This formula has been found to represent the 

 dispersion of most transparent substances. 



I have applied this formula to the region of the spectrum for which 

 the nitroso is most transparent, using the data obtained with the sjjec- 

 trometer and prisms of comparatively large angle. 



The position of the centre of the absorption band can be calculated 

 from three observed values of the refractive index, n u n 2 , n 3 , for wave- 

 lengths A x , A 2 , A 3 , from the formula, 



_ V(A,'-V)-V(X,'-X.-)C wUch = nj-nl 



This formula gives A' = 43.1 for the centre of the absorption band, a 

 value agreeing closely with the observed value. Further, 



, ( W , 3 - Wl 3 ) (A, 2 - V 2 ) (A 3 2 - A' 2 ) 



m 



2 v 2 



m = n 2 J 



A' 2 (A/ 2 - A 3 2 ) 

 m! A, 2 



A 2 2 - A' 2 



The values found for m' vary slightly with the region of the spectrum 

 in which the value of n and A are chosen. This indicates the presence 

 of infra-red absorption bands. The mean value found was 



m' = 0.53 

 m = 2.13 



If it had turned out that m equalled one, the inference would have 

 been that the absorption band at A = -13 was the only one present. The 



