I 



( 



and Surface- Colour of yitro so -dimethijU aniline. 109 



For a medium having but a single absorption-band, the 

 formula takes the form 



This formula can, I think, be applied to sodium vapour, 

 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 considered constant. The 

 formula then takes the form 



in which m^ni'' + 1. This formula has been found to repre- 

 sent the disi)ersion 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 spectrometer 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 

 >h) ^^2? '^3 for wave-lengths \i, \.2, A-3 from the formula 



, _ X3'(Xi^-V)-V(V-V)C 



in which 



C = 



n^^ — ni' 



This formula gives X' = 43*l for the centre of the absorption- 

 band, a value agreeing closely with the observed value. 

 Further, 



X,2-V"^ 



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

 the spectrum in which the values of n and \ are chosen. 

 This indicates the presence of one or more absorption-bands 

 in the infra-red. The mean values found were 



»/= 0-53 

 ni = 2*13. 



This should make the dielectric constant 2'66, if no infra-red 



