104 Prof. Wood on the Dispersioji^ Ahaorj^tio^i, 



light at various angles o£ incidence through a Nicol prism ^ 

 an attempt was next made to secure more reliable data by 

 means of the method of crossed prisms. A prism of small 

 angle, held in a clamp, was mounted with its refracting edge 

 in a horizontal position, immediately behind the prism of the 

 quartz spectrograph. A fine platinum wire immediately 

 below the edge of the prism, when heated by a current of 

 suitable strength, kept the nitroso in a fluid condition. A 

 system of sm.all screens was arranged so that the light could 

 either be made to pass through the nitroso prism, or through 

 a small clear space in the quartz plates immediately above it, 

 thus photographing the deviated and undeviated spectra one 

 above the other. The current strength was so adjusted as to 

 keep the temperature of the prism as nearly as possible at the 

 melting-point of the nitroso. It was possible in this way to 

 give exposures of an hour or more^, and obtain a photographic 

 record of the dispersion-curve from the orange down to the 

 extreme end of the ultra-violet. The cadmium spark was 

 employed as a source of light, and the slit of the spectrograph 

 was diaphragmed down to a length of less than a millimetre 

 in order to obtain very narrow spectra. As the conditions 

 necessary for success were determined by repeated experi- 

 ments, prisms of larger angle were used, and the two spectra, 

 which overlapped in the first experiments in the region where 

 the refractive index had a low value, were completely sepa- 

 rated. I found that a great improvement resulted from care- 

 fully grinding the edge of the quartz plate, which rests against 

 the other plate, perfectly straight with fine emery. It was 

 not until this expedient was adopted that the krger angles 

 became possible. The deviated spectrum is quite sharp 

 except close to the edge of the absorption-band, where 

 absorption produces a broadening of the image by reducing 

 the effective width of the beam of light, as is always the case 

 with strongly absorbing prisms. It is even possible to follow 

 the general trend of the dispersion-curve right through the 

 band of metallic absorption, though the broadening resulting 

 from diffraction is, in this region, so great that accurate 

 measurements were impossible. The continuity of the curve 

 is shown, however_, better than I have ever seen it in any 

 photograph. To deduce numerical values from the photo- 

 graphs, the distances between the spectra were measured with 

 a dividing-engine at the principal cadmium lines. From 

 these distances the actual angular deviations were calculated, 

 making proper allowance for the fact that the plate stood at 

 an angle — i. e., the focal length of the lens of the spectrograph 

 was very different in the different regions of the spectrum. 



