54 PROCEEDINGS OF THE AMERICAN ACADEMY. 



spectrum. The remarkable dispersive power of the nitroso is at once 

 apparent when we compare the two curves, and contrast it with that 

 of the bisulphide, which has the highest dispersive power of any 

 substance in common use, 



Over the region of the spectrum given above, the nitroso can be 

 considered as a transparent substance, and the dispersion formula for 

 transparent substances can be applied to it. From three values of n 

 and the corresponding values of A the value of X' , the centre of the 

 absorption band, can be calculated. 



In the case of selenium, which has a dispersion curve resembling that 

 of the nitroso, the absorption appears to increase steadily from the yellow 

 down to the extreme ultra-violet, making it impossible to determine ex- 

 perimentally the centre of the absorption band which is chiefly responsible 

 for the dispersion. Applying the formula for transparent substances to 

 the values found for selenium, I found that if we assume a single absorp- 

 tion band, its centre must be at wave-length .00056. To account for 

 the continued absorption as we pass down the spectrum, we have only to 

 assume that there is a series of bands of which the calculated one is the 

 first member. I expected that the nitroso would behave in a similar 

 manner, but found on examining its transmission that the absorption, 

 which commenced at A = .0005, ended quite abruptly at about .00037, 

 the substance transmitting the ultra-violet almost down to the last cad- 

 mium lines. This property of the substance enabled me to prepare 

 screens transparent only to ultra-violet light, which I have described in 

 a previous paper.* 



The transparency of the nitroso on the ultra-violet side of the absorp- 

 tion band is, however, very much less than on the green side, as I soon 

 found in endeavoring to measure the dispersion by crossing a prism of 

 the substance formed between quartz plates, with the prism of a quartz 

 spectrograph. No trace of the spectrum on the more refrangible side 

 of the absorption band appeared on the photographic plate. After a 

 number of failures, which obviously resulted from the insufficient trans- 

 parency of the nitroso prism, combined with the necessarily short ex- 

 posure, I abandoned this method, and made some rough determinations 

 of the ultra-violet dispersion, by observations on the angle of maximum 

 polarization for these wave-lengths. The results, while not very accu- 

 rate, gave unmistakable evidence of anomalous dispersion, the refractive 

 index for wave-lengths immediately adjacent to the absorption band 



* Phil. Mag. and Astrophysical J., 1903. 



