1910-11.] Absorption of Light by Inorganic Salts. No. IV. 547 
XXXIX. — The Absorption of Light by Inorganic Salts. No. IV. : 
Aqueous Solutions of Cobalt and Nickel Salts in the Ultra- 
Violet. By R. A. Houstoun, M.A., Ph.D., D.Sc., Lecturer in 
Physical Optics, and John S. Anderson, M.A., B.Sc., Carnegie 
Scholar in the University of Glasgow. Communicated by Prof. 
A. Gray, F.R.S. 
(MS. received March 8, 1911. Read June 5, 1911.) 
In Kayser’s Spectroscopie, vol. iii. pp. 45-49, an account is given of all 
the different methods that have hitherto been employed for photometry in 
the ultra-violet part of the spectrum. Photography, phosphorescent plates, 
selenium cells, and the ionising effect of ultra-violet light have all been 
used, but with limited success, and Professor Kayser considers that the 
only really practical method is that which has been recently introduced by 
Pfltiger. Pfliiger has discovered that there is relatively an enormous amount 
of energy in the ultra-violet spectrum of the electric spark produced by the 
discharge of a condenser between metal electrodes. He therefore uses the 
electric spark as a source, and takes the deflections with a thermopile and 
galvanometer, just as in the infra-red. He finds the deflections to be 
wonderfully steady, considering the inconstant nature of the spark. 
Pflliger’s method requires a powerful induction coil and a very sensitive 
galvanometer. The universal method in the ultra-violet is photography. 
The latter involves much less expense. We felt, therefore, that it would be 
of great advantage if a simple and practical spectrophotometer, suitable for 
use throughout the ultra-violet, could be designed and built. We set our- 
selves this task, and after some trouble have been completely successful. 
The density of the image on a photographic plate is not proportional to 
the time of exposure, even within the range of the plate. Nor, when two 
beams of light produce the same density on the same plate in different times, 
are their intensities inversely proportional to the time of exposure. Work 
of a kind can be done in such cases by making auxiliary experiments, but 
it is unsatisfactory. Only when two beams of light of the same wave- 
length fall on adjacent parts of the same photographic plate and produce 
equal blackening in the same time can we say that their intensity is equal. 
This principle is essential. It was adopted from the first, and made the 
basis of all our experiments. 
The first step was to find a means of diminishing the intensity of a 
