SELECTION OF MATERIAL, APPARATUS, ETC. 5 



exposure was finished. Water is, however, the solvent generall}' used, and 

 the easiest one to manage. It is moreover free from ultra-violet absorp- 

 tion, which is not true of the majority of the other solvents available, and 

 all dyes which can be dissolved in water can be used for staining gelatin 

 films. The gelatin can be dissolved in the solution of the dye and clean 

 glass plates flowed with the warm solution, or an unexposed photographic 

 plate, after preliminary treatment with thiosulphate of soda and thorough 

 washing, ma}- be stained with the solution of the dye. It is probable that 

 the position of the absorption bands is the same in gelatin as in water, for 

 the indices of refraction of the two media are very nearly the same. 



ABSORBING MEDIA. 



Because of their great variety, strong selective absorption, and general 

 interest, aniline dves and their related organic compounds were selected as 

 best suited for the study contemplated. 



DISPERSING SVSTEM. 



In order to obtain reasonably normal spectra a spherical, concave, 

 speculum grating, whose radius of curvature was 98.3 cm., was used. For 

 the first order spectra and for short photographic exposures the astigmatism 

 of the reflector did not produce deleterious effects. This was determined 

 by actual measurements. The length of one line of the ruling was 1.96 cm., 

 and the assemblage of lines covered 5.36 cm. The spectroscopic resolving 

 power was 21,250 (2.125 inches with 10,000 lines per inch). The incon- 

 venience of superposed higher orders will be mentioned later on. To obtain 

 a general idea of the normality of the spectrograms and of the linear dis- 

 persion it may be stated that, by calculation one millimeter the center of 

 which was at 214.7.",", or 399.4,",^, or 656.3,","., covered 25.77, 25.84, and 

 25.71 A. U., respectively, for the spectrum was designed to be normal at 

 the air line 399. 4,"/^. 



PHOTOGRAPHIC MATERIAL. 



Because of the short radius of curvature of the focal surface (about 49 

 cm.) celluloid films were employed in most cases. The films used through- 

 out were M. A. Seed's "L-ortho cut negative films," size 5 by 7 inches. 

 The emulsion is by no means equally sensitive over the field of wave-lengths 

 studied, i. e., from 0.2," to 0.63,". The chief maximum of sensitiveness is 

 in the 3'ellow, about 0.56,". A much weaker maximum is near 0.49,". The 

 middle of the less sensitive intervening region is ver}' roughly 0.52//. 



For the short exposures given throughout, these films are not appreciabl}- 

 influenced b}- wave-lengths longer than about 0.61,". The resultant effect of 

 the Nernst glower and the Seed emulsion is best understood by referring to 

 fig. 102, plate 26, for which the times of exposure were, in order, 2 seconds, 

 5 seconds, 15 seconds, 30 seconds, i minute, 2 minutes, and 3 minutes. 



