446 
PHYSICS: NICHOLS AND HOWES 
In the collimator of a spectroscope a doubly refracting rhomb was so 
mounted as to give vertically displaced images of the slit, and the length 
of the slit was adjusted so as to render these images contiguous but not 
overlapping. Two spectra one above the other were thus produced 
polarized vertically and horizontally and when the crystal to be studied 
was mounted before the slit and carefully adjusted by rotation until its 
planes of polarization were Hkewise vertical and horizontal these spectra 
gave the fluorescence and absorption for the green and white components 
respectively. A suitable light filter of deep blue glass which was opaque 
to the entire region occupied by the fluorescence so that the bands 
appeared upon a black background was placed in the path of the incident 
hght. It transmitted light of the wave-lengths necessary to excitation, 
however, and this included the region of the absorption spectrum, so that 
fluorescence bands and absorption bands were simultaneously visible 
in both spectra. 
Complete separation of the two polarized components, even when the 
adjustments are accurately made, occurs only when the crystal is per- 
fect and many specimens were tested and discarded before one suitable 
for study was found. The best results were obtained with small thin 
crystals newly formed and immediately protected from the air by a layer 
of Canada balsam. 
Comparisons of the fluorescence bands by the method just described 
show that in the various groups each band consists of two components; 
thus confirming in general the conclusions reached by the study of the 
unpolarized fluorescence at +20°C. and —185°. These components 
moreover are polarized at right angles to each other. The stronger 
fluorescence, as might perhaps be expected, is polarized in the green 
direction, corresponding with the greater absorption. 
The location and roughly approximate relative intensity of the 
fluorescence of each group is indicated in figure 1, in which for a single 
typical group the bands of the unpolarized spectrum are shown and, 
below, the green and white components. From this figure it will be seen 
that all except the band b appear as oppositely polarized doublets. 
This band in the green spectrum is absent or too dim for detection. We 
know however that at — 185° a component at the position ^ takes the 
place of b hence it is probably only a question of insufficient brightness. 
The component dw is so dim as to be seen only with great difficulty and 
the same is true of aw. 
For purposes of measurement photographs were made of the spectra 
of this compound and also of the fluorescence and absorption of crystals 
of potassium uranyl chloride, caesium uranyl chloride, and rubidium 
