6 ATLAS OF ABSORPTION SPECTRA. 



Various schemes to make the resultant action more uniform were tried 

 and other makes of films were tested, but no improvement on the simple 

 combination of the Seed emulsion and the Nernst glower resulted, therefore 

 the}' were used almost exclusively. The Seed films are good in the ultra- 

 violet as is shown by the fact that with an exposure of 5 minutes the alumin- 

 ium line at 185///* was clearly recorded. To see if appreciable shifts in the 

 apparent positions of the absorption bands were produced by the yellow 

 maximum and the green minimum of the Seed films, negatives of the same 

 absorbing medium, under exactly the same conditions, were taken on sev- 

 eral different makes of films and plates which did not exhibit maxima and 

 minima of sensitiveness for the same wave-lengths. Also, other and inde- 

 pendent tests of this possible source of error were made. The conclusion 

 was that no noticeable displacements of the bands were caused. However, 

 in the cases of brown and other visibly colored solutions, exhibiting weak, 

 general absorption, the observer of the appended positives must be careful to 

 distinguish between true absorption and the spurious effects in the vicinity 

 of 0.52,'/. 



In photographing bands in the orange and red, Cramer "Trichromatic" 

 plates were found to be the best and hence they were used. The plates 

 being plane they had to occupy a mean position with respect to the focal 

 surface of the grating. Since only a comparatively small region of wave- 

 lengths was thus recorded, no measurable errors were introduced. In fact, 

 in the region considered, the second order ultra-violet of a discontinuous 

 spectrum taken on a film and on a plate could be superposed fine for line. 



The developer used was a simple hj'drochinone solution made up 

 according to Jewell's formula.* 



SOURCES OF LIGHT. 



For wave-lengths from "above" 0.65,". to "below" 0.326//., and for 

 exposures of about one minute, the Nernst glower was found to be the most 

 satisfactory. Prevailing circumstances made desirable the use of 104 volt 

 glowers on a circuit carrying about 133 cycles. The emissivity of the Nernst 

 lamp varies so very greatly with the e. m. f. impressed upon its terminals 

 that it was obligatory to keep in series with the glower a Thomson A.C. 

 ammeter having a range from zero to two amperes and graduated directly 

 to 0.02 ampere. Fluctuations of more than 0.02 ampere invariably resulted 

 in a 'spoiled photograph, consequently boxes containing variable metallic 

 resistance were maintained in series with the ammeter and thus, in spite of 

 large changes in the load on the dynamo, due to other experimental circuits, 

 it was possible to prevent the effective current in the filament from changing 



* L. E. Jewell. Astrophys. Jour., v. xi, 1900, pp. 240-243. 



