PHYSICS: E. L. NICHOLS 
199 
200 kinds of trains altogether, represented in their connection by graphs, 
so that a glance furnishes intuitive evidence of their essential difference. 
For the groupless systems Miss Cummings gives a table of the indices 
which represent the sequences of each system, and comparison is not 
difficult. She exhibits also a table of the four varieties of interlacing 
of pairs, as distinguished by Mr. Cole, showing how many of each kind 
are found in each system. These latter data alone are found, in eight 
cases, to fail to discriminate two systems actually different. Perfect 
discrimination would almost certainly be possible by the use of a double 
entry table, 15 by 15, showing the exact distribution of tetrads, hexads, 
oktads, and dodekads. 
NEW DATA ON THE PHOSPHORESCENCE OF CERTAIN 
SULPHIDES 
(DISCUSSING MEASUREMENTS BY DRS. H. E. HOWE. H. L. HOWES AND 
PERCY HODGE) 
By Edward L. Nichols 
DEPARTMENT OF PHYSICS, CORNELL UNIVERSITYi 
Read before the Academy, November 14, 1916 
Ph. Lenard to whom we owe extended studies of the class of highly- 
phosphorescent substances known as the Lenard and Klatt^ sulphides, 
describes^ the spectrum of the emitted light as consisting of a single 
broad band in the visible spectrum. This band which appears single 
in most cases, as viewed with the spectroscope, does not however con- 
form to the recognized criteria. The marked difference between the 
color of fluorescence and that of phosphorescence and the changes of 
color during decay, suggest over-lapping bands. As shown by E. 
BecquereP the color of the emitted light varies with the wave length 
of the exciting rays. His observations apply, it is true to sulphides of 
barium, calcium and strontium not identical with the preparations of 
Lenard and Klatt, but belonging to the same class. In a recent paper^ 
I gave more direct evidence of the existence of more than one band in 
the spectra of these substances. 
In their original paper^ Lenard and Klatt depicted these spectra as 
complex instead of single; but in both the earlier and the later papers 
attention is given rather to the mode of excitation than to the character 
of the phosphorescent light itself and the regions of excitation in the 
violet and ultra violet are carefully mapped. 
Significance of the Bands of Excitation. — It seemed probable that these 
regions of maximum excitation, the positions and appearance of which 
