178 HYDRATES IN AQUEOUS SOLUTION. 



THE MAKING OF THE PHOTOGRAPHS. 



PHOTOGRAPHIC MATERIAL.* 



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

 cm.), celluloid films were employed in the majority of cases. The films 

 used throughout 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 photographically, i. e., from 0.20,u to 0.63,". The 

 chief maximum of sensitiveness is in the yellow at about 0.56,". A weaker 

 maximum is near 0.49//.. The middle of the less sensitive intervening 

 region was very roughly 0.52,".. For the short exposures given through- 

 out, these films are not appreciably influenced by wave-lengths longer than 

 about 0.61,"-. Consequently, the dark regions extending from about 0.61/* 

 to the longer wave-length end of such spectrograms as were gotten by 

 the use of the Seed films, do not denote absence of light due to the absorp- 

 tion of some given solution, but they make manifest the lack of sensitive- 

 ness of the emulsion to the portion of the spectrum under consideration. 

 The resultant effect of the Nernst glower and the Seed emulsion is best 

 understood by referring to plate 1 (6), for which the times of exposure were, 

 in order, 2 seconds, 5 seconds, 15 seconds, 30 seconds, 1 minute, 2 minutes, 

 and 3 minutes. 



Various schemes to make the resultant action from the light of the Nernst 

 glower and the Seed emulsion more uniform were tried, and other makes 

 of films were tested, but no improvement on the simple combination of the 

 source of light and emulsion just named was found. In fact, other photo- 

 graphic films did not act as well, on the whole, as those furnished by the 

 Seed Company. The films used were good in the ultra-violet, as is shown 

 by the fact that with an exposure of five minutes the aluminium line at 

 about 0. 185,'j. was clearly recorded, in spite of the fact that the grating of 

 the spectrograph was employed to disperse the light. When inspecting the 

 photographs of absorption spectra, care must be taken to distinguish between 

 possible spurious effects, arising from the maxima and minima of sensitive- 

 ness of the Seed films on the one hand, and the phenomena of true absorp- 

 tion on the other. Observe the apparent band in the green as shown by 

 plate 1 (6). 



As a check on the results obtained with the films, as well as to fill in the 

 gap between about 0.59/Jt and the nearer end of the field of view of the 

 spectrograph, Cramer trichromatic plates were used, since they were found to 

 be the more satisfactory. The plates, being plane and too rigid to be curved 

 around into the focal surface of the grating, had to occupy a mean position 

 with respect to this surface. Since only a comparatively small region of 



* Uhler and Wood: Atlas of Absorption Spectra, 



