240 Mechanical Action of Light. [April, 
is as 288 to 289. They therefore balance each other 
as nearly as possible. Similarly I can balance a gas- 
light against a candle. I have a small gas-burner here, 
which I place 28 inches off on one side, and you see it 
balances the candle 12 inches off. These experiments show 
how conveniently and accurately this instrument can be used 
as a photometer. By balancing a standard candle on one 
side against any source of light on the other, the value of 
the latter in terms of a candle is readily shown ; thus in the 
last experiment the standard candle 12 inches off is balanced 
by a gas-flame 28 inches off. The lights are therefore in the 
proportion of 12 2 to 28% or as 1 to 5*4. The gas-burner is 
therefore equal to about 5J candles. 
In practical work on photometry it is often required to 
ascertain the value of gas. Gas is spoken of commercially 
as of so many candle-power. There is a certain “ standard ” 
candle which is supposed to be made invariable by Aft of 
Parliament. I have worked a great deal with these standard 
candles, and I find them to be among the most variable 
things in the world. They never burn with the same lumino- 
sity from one hour to the other, and no two candles are 
alike. I can now, however, easily get over this difficulty. 
I place a “ standard ” candle at such a distance from the 
apparatus that it gives a deflection of 100 degrees on the 
scale. If it is poorer than the standard, I bring it nearer ; 
if better, I put it farther off. Indeed any candle may be 
taken ; and if it be placed at such a distance from the appa- 
ratus that it will give a uniform deflection, say of 100 divi- 
sions, the standard can be reproduced at any subsequent 
time ; and the burning of the candle may be tested during 
the photometric experiments by taking the deflection it 
causes from time to time, and altering its distance, if needed, 
to keep the deflection at 100 divisions. The gas-light to be 
tested is placed at such a distance on the opposite side of 
the pith bar that it exadtly balances the candle. Then, by 
squaring the distances, I get the exact proportion between 
the gas and the candle. 
Before this instrument can be used as a photometer or 
light measurer, means must be taken to cut off from it all 
those rays coming from the candle or gas which are not 
adtually luminous. A reference to the spedtrum diagram 
(Fig. 5) will show that at each end of the coloured rays, 
there is a large space inadtive, as far as the eye is concerned, 
but adtive in respedt to the production of motion — strongly 
so at the red end, less strong at the violet end. Before the 
instrument can be used to measure luminosity, these rays 
