and Surface -Colour of jSitroso-dimetliyl-aniUne, 111 



the spectrum between wave-lengths 23 and 29 until we know 

 the value o£ the extinction-coeflicient in this region. 



I feel certain that there are absorption-bands in the infra- 

 red, not only on account of the indications which the 

 dispersion-formula gives, but also because when working with 

 prisms of large angle 1 found that a fairly strong absorption 

 occurred in the extreme visible red. It was not sufficiently 

 intense to make measurements impossible^ and whether it was 

 due to the presence of a weak diffused band similar to the 

 one at X = 25, or to a strong band of metallic absorption, I 

 was unable to determine, the observations being confined to 

 the extreme edge. I expect to investigate this point as soon 

 as our apparatus for the study of the infra-red region of the 

 spectrum is in working order. I hope to be able in the near 

 future to make a determination of the dielectric constant of 

 the substance. With these points thoroughly investigated, a 

 more rigorous application of the dispersion -formula to the 

 results will be possible. I feel less hopeful of getting 

 satisfactory data regarding the absorption, but shall at all 

 events make the attempt. 



The Dispersion of Toluine. 

 In examining the ultra-violet absorption of some organic 

 solvents, I found that toluine has a fairly strong and narrow 

 absorption-band at wave-length '00027, in addition to the 

 band of metallic absorption below '00020, which is the one 

 which chiefly influences the dispersion of the substance. It 

 occurred to me that it would be interesting to determine the 

 dispersion of the substance through this region, and see to 

 what extent the curve was modified by the weaker band. So 

 far as I was able to find, the ultra-violet dispersion of toluine 

 has never been determined, which is not very surprising, 

 since only very acute prisms transmit anything below the 

 band above mentioned. The same method was employed 

 as in the case of nitroso-dimethyl-aniline, namely, the crossed- 

 prism method adapted to the quartz spectrograph. The 

 dispersion was first measured in the visible spectrum Avith 

 a hollow prism of 60*^ angle. Small prisms were then made 

 of quartz plates with the toluine between them, the angles 

 varying from two to six degrees. The angles were deter- 

 mined from the photographs of the deviated sj^ectra obtained 

 with the quartz spectrograph, using the data obtained 

 wdth the spectrometer in the blue region of the spectrum. 

 Fig. 9 (PI. VIII.) shows one of the deviated spectra, the ultra- 

 violet being to the left, and illustrates well the general form 

 of the dispersion-curve in this region. Figs. 10 and 11 show 

 the deviated and undeviated spectra, the one above the other,. 



