PLATES. 183 



that no appreciable error has been introduced in this manner. Moreover, 

 shifts in wave-lengths due to the motion of the film-holder and its guides 

 or ways are present in most of the figures. These displacements can be 

 measured and allowed for in the manner suggested above. It must be 

 borne in mind that the solutions studied present such washed-out limits to 

 most of their regions of absorption, that it is not possible to assign numbers 

 to the wave-lengths of these boundaries, which would be nearly as accu- 

 rate as the determination of the relative shifts of the parallel photographic 

 strips. Moreover, with bands of absorption of the type just referred to, 

 their apparent ends would be displaced in wave-length by various experi- 

 mental causes, such as variations in the intensity of incident light, changes 

 in the time of exposure to the radiations, changes in the duration of develop- 

 ment, variations in the temperature and composition of the developer, 

 etc. The best that could be hoped for was to obtain parallel spectrographic 

 strips of the solutions of a given series, which exhibit the spectral changes 

 caused by the solutions themselves, and only by the solutions; extreme 

 care being taken to keep all other conditions as nearly constant as possible. 

 The figures therefore show the relative changes in the absorption spectra 

 of the solutions of one set with accuracy. Whenever there was the slight- 

 est possibility of spurious results arising from the chief maxima and min- 

 ima of sensibility of the seed emulsion, the results were checked by taking 

 additional plates with the Cramer trichromatic emulsion, which has not 

 the same sensibility curve as the Seed film. In all cases the series of eye 

 observations with the spectroscope not only supplement the photographic 

 data in the red, but they also serve as an additional qualitative check on 

 the entire visible region. 



For comparatively accurate estimations of wave-lengths, one comparison 

 spectrum of the emission lines of the spark has been reproduced in each pho- 

 tographic figure. The most intense and sharpest lines have been numbered 

 in plate 1 (a). This plate serves as a key to the table of wave-lengths given 

 a little later.* 



Since the spark lines of cadmium, zinc, oxygen, and nitrogen are not equally 

 spaced, the comparison spectrum mentioned above, together with its key 

 and table, is very inconvenient for most purposes of reference. Conse- 

 quently, a numbered linear scale of wave-lengths has been printed along 

 by the side of each complete spectrogram. Although it is not professed 

 that these scales are as accurate as they are suggestive and convenient, 

 nevertheless, it is only fair to call attention briefly to the chief errors pos- 

 sessed by them. As is well known, a concave grating gives spectra which 

 depart more and more from normality as we recede from the principal 

 axis of the reflector on which the lines of the grating are ruled. Therefore, 



* Uhler and Wood: Atlas of Absorption Spectra. 



