200 
PHYSICS: E. L. NICHOLS 
had long since been depicted by Becquerel, were due to the presence of 
absorption bands. Dr. H. E. Howe, who was employed last summer 
in the study of ultra violet absorption spectra, was kind enough to test 
this hypothesis. 
In his experiments the phosphorescent substance was exposed to the 
continuous ultra violet spectrum of the powerful submerged aluminum 
spark described by Henri^ and subsequently employed by Howe^ in 
his study of absorption spectra. The bands of excitation (Erregungs- 
bande of Lenard) were thus located and with these absorption bands of 
the phosphoresent substance, obtained by reflection, were shown to 
coincide in position and extent. In the case of a barium sulphide with 
lead as an active metal, the crests of the bands of excitation were at 0.380)u 
and 0.335/1. Lenard gives for a sulphide of similar composition, 0.377/i 
and 0.332/x. 
The crests of the absorption bands are at 375ju and 332/x. Similar 
coincidences were established in the case of Sr, Zn and Sr, Pb sulphides 
and the relation is therefore probably a general one corresponding to 
that already established in the case of the selective activity of infra 
red rays upon the phosphorescence of zinc sulphide,^ where the maximum 
effect was found in regions of maximum absorption. 
Spectrophotometric Measurements. — A detailed spectrophotometric 
study reveals widely varying degrees of complexity in the spectra of 
these sulphides. 
Dr. H. L. Howes kindly made for the writer very careful measurements 
of three characteristic compounds. Settings were made at intervals 
of 50 Angstrom units throughout the spectrum. 
The curves thus obtained show the existence of many overlapping 
bands so nearly merged that to the eye the appearance is that of a single 
simple band. There is moreover a distinct suggestion of a systematic 
relation. 
Taking the relative frequencies, i.e., reciprocals of the approximate 
wave lengths of the crests (1 //z X 10^) it is found that the over-lapping 
bands are members of one or more series of constant frequency interval; 
which interval varies for the different sulphides. 
Whether the spectra under consideration are to be regarded as con- 
sisting of a single band or of more than one band is not a question of 
complexity of structure. 
Any system however complex which behaves as a unit under varying 
conditions of temperature, mode of excitation, etc., all the components 
being affected in like manner, may be considered as a single band in the 
sense in which that term has been used by Lenard. We have a striking 
