90 Dr. ±\ D. Foote and Dr. W. F. Meggers on Atomic 



ftO 



beneath by an intense beam of light from a tungsten-ribbon 

 lamp. Light transmitted by the portion of the plate, the 

 image of which was adjacent to that of the photometer-lamp 

 filament, was matched against the filament brightness by 

 adjusting the current through the lamp. The current read- 

 ings are readily translated into measurements of photo- 

 graphic density, correcting, of course, for the " fog " of the 

 gelatine films. 



Several series of exposures from 2 to 300 seconds were 

 made at different times on the 6-volt caesium arc, and density 

 measurements of the spectral lines showed that the dicyanin- 

 stained plates have approximately the same development 

 factor (gamma = 1*45) for all wave-lengths from 4555 A to 

 8521 A. To calibrate the plates in terms of absolute intensity 

 of the source, series of exposures of different durations were 

 made by sighting on a black body, the temperature of which 

 was measured by an optical pyrometer. The D-lines of 

 sodium were superposed on each spectrum as fiducial marks. 

 The densities of the plates for various wave-lengths were 

 measured with the micro-photometer, and from the i; charac- 

 teristic curves " of the plates and the computed distribution 

 of energy in the spectrum of the black body, the photo- 

 graphic density as a function of wave-length was corrected 

 for an equal energy distribution in the spectrum, a typical 

 curve thus obtained being illustrated by fig. 3. With the 

 general characteristics of dicyanin-stained plates thus deter- 

 mined, the relative absolute intensities of lines in the csesium- 

 arc spectra were obtained from their measured photographic 

 densities and the durations of exposure. Since relatively 

 long exposures were required to photograph the spectra of 

 the low-intensity arcs, the reciprocity law of the photo- 

 graphic plate (density = function of intensity X time) was 

 corrected* by giving the exposure time an exponent of 0*8. 



III. Eesults. 



Fig. 4 shows the logarithms of the actual intensities of 

 various lines emitted in the caesium arc plotted against 

 applied accelerating potential. At slightly below 3 volts the 

 entire line spectrum appears, only a few of the lines of 

 which are shown in the figure. It is noted that the intensity 

 of the lines A, 8521 and X8943 persists at voltages below 

 which the intensity of all other lines vanishes. This is more 

 clearly illustrated by the following table of ratio of intensity 



* Schwarzschild, Photographische Correspondenz 1899, p. 109. 



