and Surface- Colour of JS^'itrosO'dimethjl-aniline. 105 



The anorle of the prism was calculated from the deviations in 

 the yellow and green, for which region the refractive index 

 had been already determined with considerable accuracy. In 

 spite of the rather crude method, the results obtained with the 

 different prisms agreed surprisingly well. 



It will be seen from the nhotographs, some of which are 

 reproduced in fig. 3, Plate VIIL, that the deviated and un- 

 deviated spectra come together at the ultra-violet edge of the 

 principal absorption-band, indicating a refractive index not 

 much above unity. 



(a) deviated spectrum obtained with prism of very small 

 angle; (h) (c) (d) deviated and undeviated spectra obtained 

 with prisms of larger angle. The continuity of the curve 

 through the absorption-band can be seen in (a) and {h), also 

 in (^), which was enlarged from {fi). The ultra-violet is to 

 the left in the first four of these figures. 



The prism-angles in these experiments varied from 20^ to 2°. 



It was found possible to get a fairly accurate determination 

 of the refractive index for wave-length 48 from these photo- 

 graphs, the mean of several determinations giving 2-2^ as 

 the value. The values found on the ultra-violet side of the 

 band are given in the form of a curve in Plate YII. This 

 portion of the curve is especially interesting, as the effect of 

 the ultra-violet absorption-bands is most strikingly shown. 

 The weak band at wave-length 25 causes a dip in the curve^ 

 which has a maximum at \ = 28'5, after which it again rises 

 rapidly as it approaches the heavier absorption-band situated 

 not far from X = 20. 



On this same plate I have given the dispersion in the 

 visible spectrum, together with the value ?? = 8'85 for wave- 

 length X=43 the centre of the absorption-band, which was 

 determined from the constants of elliptical polarization. The 

 trend of the curve within the absorption-band I have sketched 

 in free-hand, as well as the general form and position of the 

 absorption-bands. 



I have already shown that the reflecting power of the 

 nitroso for the wave-lengths immediately adjoining the ab- 

 sorption-band on the ultra-violet side is exceedingly small, 

 which is not surprising when we consider that the refractive 

 index for this region is not very different from air. If now 

 we consider a quartz-nitroso surface instead of an air-nitroso 

 surface, we should expect fairly strong reflexion at this point, 

 the nitroso acting as the rarer medium, while further down 

 the spectrum, at about wave-length 29, where the indices of 

 the quartz and nitroso are very nearly the same, we should 

 expect scarcely any reflexion at all. 



