318 
PROFESSOR W. Gr. ADAMS AND MR. R. E. DAY ON THE 
Numerous experiments were made in order to compare the effects of exposure to light 
emanating from various sources. For this purpose a platinum spiral or a piece of 
platinum gauze, with the substance upon it whose light was to be tested, was inserted in 
the flame of the Bunsen burner, care being taken not to expose the selenium to the light 
and heat coming from the Bunsen burner itself and the lower part of the flame. Sele- 
nium, barium, thallium, sodium, salts of strontium, and sal ammoniac were thus employed, 
and the general character of the results obtained showed clearly that the effect on the 
selenium depended much more on the illuminating power of the light than on the source 
whence it was derived. 
From the experiments already described we may fairly conclude that the luminous 
rays are the cause of this peculiar behaviour of selenium, and that the dark-heat rays 
and the chemically active rays have very little, if any, share in producing it. 
The Change in the Resistance is exactly as the Square Root of the 
Illuminating Power. 
It then became a matter of interest to know in what way the amount of change in 
the resistance of selenium depended upon the illuminating power of the light. 
It was found that, on exposing this bar of selenium to a constant source of light at 
different distances, the change in its resistance, on exposure for 10 seconds (as measured 
by the deflection of the galvanometer-needle), was almost exactly inversely as the distance 
from the source of light, that is to say, directly as the square root of the illuminating 
power. This law was found to hold whether the source of light was 1 candle or an 
Argand lamp of an illuminating power equal to 16 candles. 
Taking the mean of a number of experiments, all of which agreed pretty well toge- 
ther, the deflections at the several distances were : — 
At | metre. At \ metre. At 1 metre. At 2 metres. 
With Argand lamp 170 83 39 
,, candle 41 18 8 
„ „ 82 39 18 8 
Another set of experiments with the candle and the Argand lamp, which in this case 
had an illuminating power equal to that of 12 candles, both being at the distance of 
1 metre from the selenium, gave the following results : — 
With the candle the deflection was . . 19 divisions in 10 seconds. 
„ „ Argand lamp the deflection was 66 „ ,, ,, 
In another series of experiments the distance of the selenium bar from the Argand 
burner remained unaltered, hut the illuminating power was altered and at the same 
time measured by a Bunsen photometer : — 
