26 
PHYSICS: D. L. WEBSTER 
Proc. N. a. S. 
transformation the Xi{a,^) be the same, then corresponding to those 
orders is to be inferred the existence of the derivatives 
j j 
j j 
If these are to agree the commutator must vanish at all points actually 
occurring in the transformation; a similar limitation in the meaning of 
"identical" may be understood in the proof of sufficiency. 
THE INTENSITIES OF X-RAYS OF THE L SERIES 
IT THE CRITICAL POTENTIALS OF THE PLATINUM LINES 
By David L. Webstkr 
Department of Physics, Massachusetts Institute of Technology 
Communicated by Edwin B. Wilson. Read before Academy, November 10, 1919 
Introduction. — This work is a continuation of that of Webster and Clark, 
reported in these Proceedings^ in 1917. Part of the present work was 
done at Harvard University with the apparatus described in the earlier 
paper, and part with my new apparatus at the Massachusetts Institute of 
Technology. The object in view is the investigation of the laws relating 
intensity to potential, for the L-series lines, for the purpose of comparison 
with current theories of X-ray spectra, and the present paper deals with 
the determination of the critical potentials of the platinum lines of medium 
intensity, the stronger ones having been reported in the previous paper 
and the fainter ones, observed only in tungsten by Dershem^ and Overn,^ 
being so faint as to require a much more prolonged study. 
Apparatus. — ^The work at Harvard showed that with the slit widths 
needed for accurate intensity measurements it would be difficult to work 
with certainty on any lines but the strong ones, when the voltage was near 
the critical value. As the previous work of the author on the rhodium 
K series^ had shown that photography gave good results in such work it 
was decided to use it here. In this case, where the lines are many and 
scattered and faint, the best instrument seemed to be the bent mica 
spectrograph of de Broglie and Lindemann^ with which they have ob- 
tained excellent spectrograms. These show spectra of six orders, called 
first to sixth inclusive, though I think the last one, from its angle, must 
really be the seventh rather than the sixth, which must be very faint. 
The grating space is about 10 Angstroms and the third and fifth orders 
are the strongest. 
