PHYSICS: NICHOLS AND HOWES 
445 
ture of the room appear as separate bands. The relative brightness 
of the components, and also the effect of temperature upon them, varies 
greatly with different bands. 
Ammonium uranyl chloride crystalHzes in triclinic plates, some of 
which show a marked pleochroism. According to Mr. D. T. Wilber, 
to whom we are indebted for the preparation of this and the other uranyl 
compounds mentioned in this paper: 
The strong pleochroism is observed in crystals tabular parallel to the 
face c(OOl) the crystals being set to correspond with the isomorphous potas- 
sium uranyl chloride as described in Groth's Chemische Krystallographie, 
volume 1, p. 594. The transmitted light is polarized with one component 
vibrating nearly parallel to the edge between the c(OOl) face and the b(OlO) 
face. This is less absorbed, the transmitted light appearing nearly white in 
c 
\ 
r 
'4 
C 
{ 
\ 
< 
■'s 
1, 
i 
FIG. 2 
plates 1 mm. or less in thickness. The component at right angles to this is 
strongly absorbed in the blue and violet and appears deep greenish yellow even 
in much thinner layers. 
We shall designate these directions of polarization simply as white 
and green. 
When the violet rays from an electric carbon arc are transmitted 
through the sHt of a spectroscope and the fluorescence and absorption 
bands are observed through a Nicol prism, marked changes are found to 
occur as the Nicol is turned with its plane polarization parallel succes- 
sively to the green and white planes of the crystal. ^ 
The two components of the fluorescence and absorption may be con- 
veniently observed simultaneously by means of the following arrange- 
ment of apparatus which is that employed in our determinations. 
