1 875-] 
Mechanical Action of Light. 
241 
must be cut off. We buy gas for the light that it gives, not 
for the heat it evolves on burning, and it would therefore 
never do to measure the heat and pay for it as light. 
It has been found that a clear plate of alum, whilst letting 
all the light through, is almost, if not quite, opaque to the 
heating rays below the red. A solution of alum in water is 
almost as effective as a crystal of alum ; if, therefore, I place 
in front of the instrument glass cells containing an aqueous 
solution of alum, the dark heat rays are filtered off. 
But the ultra-violet rays still pass through, and to cut 
these off I dissolve in the alum solution a quantity of sul- 
phate of quinine. This body has the property of cutting off 
the ultra-violet rays from a point between the lines G andH. 
A combination of alum and suphate of quinine, therefore, 
limits the adtion to those rays which affedt the human eye, 
and the instrument, such as you see it before you, becomes 
a true photometer. 
This instrument, when its sensitiveness is not deadened 
by the powerful control magnet I am obliged to keep near 
it for these experiments, is wonderfully sensible to light. In 
my own laboratory a candle 36 feet off produces a decided 
movement, and the motion of the index increases inversely 
with the square of the distance, thus answering the third 
question — “ Is the amount of adtion in diredt proportion to 
the amount of radiation ?” 
The experimental observations and the numbers which 
are required by the theoretical diminution of light with the 
square of the distance, are sufficiently close, as the following 
figures show 
Candle 6 feet off gives a defledtion of 218*0° 
33 
12 
93 
33 
54 -o° 
33 
18 
93 
33 
2 4 ’ 5 ° 
33 
24 
93 
33 
13*0° 
33 
10 
33 
33 
77*0° 
33 
20 
33 
93 
19*0° 
3 3 
30 
33 
33 
8*5° 
The effedt of two candles side by side is pradtically 
double, and of three candles three times that of one candle. 
In the instrument just described the candle adts on a pith 
bar, one end of which is blacked on each side. But suppose 
I black the bar on alternate halves and place a light near it 
sufficiently strong to drive the bar half round. The light 
will now have presented to it another black surface in the 
same position as the first, and the bar will be again driven 
VOL. VI. (N.S.) 2 c 
