S30 
PHYSICS: E. L. NICHOLS 
The great advantage to be gained from the study of these substances 
is due to the well known fact that in their spectra the usual broad band 
of fluorescence or phosphorescence is resolved into a group of seven or 
more bands which do not overlap and that at low temperatures ( — 150° 
to —180°) these are further resolved into complexes of narrow line-like 
bands often well separated from one another. The particular salt men- 
tioned above was selected because at low temperature each group is 
unusually well resolved into at least seven such distinct bands. It 
should therefore be possible by comparing the spectrum during excita- 
tion with that immediately after, to detect changes affecting the indi- 
vidual components; something which is impossible in the case of the 
broad-banded spectra hitherto examined. 
For the purpose of such comparisons a pair of right angled prisms was 
mounted before the slit of a Hilger spectroscope as shown in figure 2. 
Light from the lower end 
of the tube A enters the lower 
half of the slit. That from 
the upper end B after two 
total reflections enters the 
upper half of the slit and we 
have two spectra one above 
the other, coinciding through- 
FiG. 3a. FIG. 3b. out, as to wavc length, but 
separated by a dark line 
formed by the lower edge of the second prism (R'). 
To compare fluorescence with phosphorescence the sectored disk was 
shifted upon its shaft until the lower end of the tube was viewed during 
excitation, the upper end immediately after (fig. 3 a). To compare the 
phosphorescence spectrum at an earlier and later stage the disk was so 
set that its position at the moment of excitation was as shown in fig. 3 b. 
By means of the reflecting prisms at the slit of the spectroscope, already 
described, the spectrum of the light emitted from region A was com- 
pared with that at B in each case. At H-20° the banded spectra were 
found to be identical in every respect, except in brightness; and the 
same was true at low temperatures where it was possible to inspect each 
of the numerous line-like bands individually. 
Of the seven homologous series distinguishable in the fluorescence 
spectrum all were present in the phosphorescent light, unshifted as to 
position and not perceptibly enhanced or diminished in relative bright- 
ness. 
The comparison was less satisfactory as regards minor details in the 
