100 Prof. Wood on the Dispersion, Absorption, 



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 X = -0005 ended quite 

 abruptly at about '00037, the substance transmitting the 

 ultra-violet almost down to the last cadmium 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 (Phil. Mag. Feb. 1903). 



The transparency of the nitroso on the ultra-^-iolet side of 

 the absorption-band is, however, very much less than on the 

 green side, as I soon found in endeavouring to measure 

 the dispersion by crossing a prism of the substance formed 

 between quartz plates, with the prism of a quartz spectro- 

 graph. Xo 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 transparency of the nitroso prism, com- 

 bined with the necessarily short exposure, 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 accurate, gave unmistakable evidence of anomalous dis- 

 persion, the refractive index for wave-lengths below the ab- 

 sorption-band ranging from I'l to about I'D. I was subse- 

 quently able to use the method of crossed prisms, by employing 

 very acute prisms, kept warm by means of an electrically 

 heated platinum wire, which made long exposures possible. 



I shall first, however, consider the polarization phenomena 

 accompanying the reflexion of light from the surface of the 

 fluid nitroso. The substance was kept in a liquid condition 

 bv means of a small cell heated bv a current of steam 

 (fig. 1. p. 101). 



On examining the light reflected at a fairly large angle 

 with a Xicol prism, it will be found to vary from light blue 

 to deep violet and purple, as the angle of incidence is in- 

 creased, the nicol being held in such a position as to refuse 

 transmission to the light polarized by reflexion. If a spectro- 

 scope is placed behind the nicol. a dark band will be seen 

 crossing the spectrum, which shifts its position as the incidence 

 angle varies. The centre of this dark band is evidently the 

 wave-length for which the angle of incidence happens to be 

 the angle of maximum polarization: or, in other words, the 

 refractive index of the substance for this wave-length is the 

 tangent of the angle of index. In the case of glass and 



