88 Dr. P. D. Foote and Dr. W. F. Meggers on Atomic 



The spectrum photographs were made on Seed 23 plates 

 stained with dicyanin. A few of the plates were sensitized 

 with Hoechst dicyanin, but most of them were prepared with 

 dicyanin made by the Bureau of Chemistry, Department of 

 Agriculture, since these American dyes appear to be equal 

 or superior to the German ones in photo-sensitizing action. 

 The process of staining has been described several times, and 

 it lias been used almost continuously in spectroscopic inves- 

 tigations at the Bureau of Standards for the past four years. 

 This experience in staining has made it possible to reproduce 

 sensitized plates which are remarkably similar, so that very 

 nearly the same spectral sensitivity is shown by different 

 plates. This uniformity is of importance in the comparison 

 of spectroscopic data obtained from different plates. 



The spectrograph was made by Carl Zeiss, and has a large 

 flint prism of 10 cm. base length and 60° angle. The colli- 

 mator and camera lenses have 5 cm. aperture and 26 cm. 

 focal length. On account of the very low intensity of the 

 low-voltage arc a spectograph with the highest attainable 

 light efficiency is advantageous. About thirty lines with 

 wave-lengths from 4555 A to 8943 A were recorded with an 

 exposure of one minute to the 6-volt caesium arc. An 

 exposure of 5 minutes showed additional lines and extended 

 the spectrum from 3878 A to 9208 A. The exposures were 

 much longer when the arc was operated at 3 volts or less. 

 Most of these averaged about 15 hours, although in some 

 cases exposures of 40 to 60 hours were employed. 



The length of the spectrum on the photographic plate was 

 about 40 mm. between 3878 A and 9208 A, and the dispersion 

 in this range was sufficient to separate all of the important 

 doublets in the various spectral series, even when a relatively 

 large slit width (0*2 mm.) was used. 



Plate I. shows a photograph of several typical exposures at 

 different voltages. Lower voltage exposures were made, but 

 were not suitable for reproduction although the original 

 negatives were measurable. It appears from visual observa- 

 tion that the first doublet of the principal series alone 

 appears below the applied potential of 2'4 volts ; but this fact, 

 as has been heretofore, with other elements, assumed con- 

 clusive, should not be accepted as proof of the existence of a 

 single-line spectrum. If the plates possessed a constant 

 sensitivity to light of various wave-lengths, the line X 8521 

 could be made to appear alone on very short exposure even 

 in a high- voltage arc, because in absolute energy it is the 

 most intense line of the spectrum. The sensitivity of the 

 plate, however, varies with the wave-length, as shown by 



