130 STUDIES IN LUMINESCENCE. 



exposure. If several spectra are photographed upon the same plate and 

 the length of exposure is the same for all the spectra, then the denseness of 

 the different negatives at any wave-length will depend only upon the 

 intensity of the incident light at that wave-length, assuming a small opening 

 of the slit to the spectroscope, uniform development, non-halation, etc. 

 Since intensity is proportional to the rate at which energy is received from 

 the source, the denseness of the different negatives at any wave-length 

 depends upon the energy received from the source at that wave-length. 

 By making several negatives of one source of light of known energy dis- 

 tribution at different distances from the spectroscope, curves can be drawn 

 showing the relation between energy received at any wave-length and dense- 

 ness of negative. If a spectrum of another light of unknown energy dis- 

 tribution is photographed on the same plate, its energy distribution can be 

 obtained by comparison of the denseness of its negative with the calibrated 

 densities. The densities can be compared by means of the light transmitted 

 through the negatives at different wave-lengths. 



If the slit opening in the spectroscope is not very small, the denseness of 

 the negative at any wave-length will not depend alone upon the energy 

 received at that wave-length, but will also depend upon the energy received 

 at wave-lengths differing but little from the wave-length in question. 

 Consequently a small dimple in a curve might be obliterated by a wide slit 

 opening. The effect of slit opening can be tested by using different openings, 

 finally using the one which experience shows to be best. 



The effect of halation is more troublesome than slit opening, and was 

 present more or less in all of the negatives. The effect of halation is to 

 broaden the band of energy distribution, but not to change the position 

 of the maximum point of the band. It is believed that halation in the 

 present experiments was not of sufficient effect to cause any serious error in 

 the curves. 



The plates were developed immediately after the exposures, which were 

 made one directly after the other, so that any error due to continued chemi- 

 cal action after exposure in the film was eliminated as far as possible. 



The photometric measurements were difficult to make because of the 

 large differences in the amount of light transmitted in different parts of the 

 negatives. At the beginning of the experiments, four settings of the pho- 

 tometer were made for each point measured. Later two settings were found 

 to be sufficient, one approaching uniformity of illumination from each 

 direction. The average of the two values calculated from the two settings 

 was taken as the true ratio of standard and transmitted lights. Remeasure- 

 ment of a set of curves never proved any exceptional accuracy, but always 

 gave the same type of energy curve with the maximum at nearly the same 

 wave-length. Consequently the measurements can be said to be substan- 

 tially correct. 



These experiments show that the energy curve of the fluorescence light of 

 Sidot blende consists of a band extending from about ^ = 0.46 to ;u = o.6o, 

 having a maximum at ju = 0.55. There may be another band situated in the 

 region of longer wave-lengths. Furthermore, the energy distribution in the 

 fluorescence light and the phosphorescence light immediately after excita- 

 tion is the same. 



