PHYSICS: NICHOLS AND HOWES 
447 
uranyl chloride prepared for us by Mr. Wilber. In photographing 
absorption bands a nitrogen filled tungsten lamp was substituted for the 
arc and a light blue filter was sometimes used in place of the deep blue 
screen already described. Owing to the very great range of intensity 
in the fluorescence bands and the great difference in the transmission, 
within the region of absorption for the white and green directions respec- 
tively it was necessary to make many exposures of varying duration in 
order to obtain a complete record of the spectra. 
When the transmitted light is polarized and parallel to one of the 
planes of vibration of the crystal, only one absorption spectrum is ob- 
served and the completeness of extinction of the other affords a delicate 
test for the adjustment of the apparatus and for the homogeneity of the 
specimen. It is however significant that the two polarized fluorescence 
spectra are always present, provided the emission is that leaving the 
-__83.2 M 83.a---^ 
k 83.2 >|f 83.2 ^ 
K 71 
k 83.2- 5k 83.2---— > 
""^--71"- 
K - 
y 83.2^ —-ik— -83.2 ^ 
-7|— 71 
^§ 
IdiOO 20100 
-71 
21100 
FIG. 3 
crystal in the direction described in an earlier paragraph, and these spec- 
tra are the same whether the exciting light is polarized in the while or the 
green direction or whether unpolarized light is used for excitation. The 
exciting light, moreover, may enter the crystal in any direction without 
affecting the character of the fluorescence spectra. ^ 
This is in accord with the general principle established by the study 
of numerous cases of fluorescence,^ i.e., that a fluorescence band is 
independent, as regards its location and character, of the nature of the 
excitation. 
It follows moreover that a beam of plane polarized light is capable 
of exciting not only the polarized fluorescence which has its vibration 
in the same plane but likewise that component of the fluorescence which is 
polarized at right angles to the exciting light. This would seem to remove 
the fluoresence of these crystals and presumably the very similar fluores- 
cence of the other uranyl compounds from the class designated by Wood 
as resonance spectra. 
