Mechanical Action of Light. 
239 
1876.] 
black and one-half white, exposed to a broad beam of radi- 
ation, will be pushed with 5J- times more strength on the 
black than on the white half, and if freely suspended will 
set at an angle greater or less according to the intensity of 
the radiation falling on it. 
This suggests the employment of such a bar as a photo- 
meter, and I have accordingly made an instrument on this 
principle : its construction is shown in the diagram (Fig. 6). 
It consists of a flat bar of pith, A, half black and half white, 
suspended horizontally in a bulb by means of a long silk 
fibre. A reflecting mirror, B, and small magnet, c, are 
fastened to the pith, and a controlling magnet, D, is fastened 
outside so that it can slide up and down the tube, and thus 
increase or diminish sensitiveness. The whole is completely 
exhausted and then enclosed in a box lined with black velvet, 
with apertures for the rays of light to pass in and out. A 
ray of light from a lamp, F, reflected from the mirror, B, 
to a graduated scale, g, shows the movements of the 
pith bar. 
The instrument fitted up for a photometric experiment is 
in front of me on the table. A beam from the eleCtric light 
falls on the little mirror, and is thence reflected back to the 
screen, where it forms a spot of light, the displacement of 
which to the right or the left shows the movement of the 
pith bar. One end of the bar is blacked on each side, the 
other end being left plain. I have two candles, E E, each 
12 inches off the pith bar, one on each side of it. When I 
remove the screens, H H, the candle on one side will give the 
pith a push in one direction, and the candle on the other side 
will give the pith a push in the opposite direction, and as 
they are the same distance off they will neutralise each 
other, and the spot of light will not move. I now take 
the two screens away : each candle is pushing the pith 
equally in opposite directions, and the luminous index 
remains at zero. When, however, I cut one candle off, the 
candle on the opposite side exerts its full influence, and the 
index flies to one end of the scale. I cut the other one off 
and obscure the first, and the spot of light flies to the 
other side. I obscure them both, and the index comes 
quickly to zero. I remove the screens simultaneously, and 
the index does not move. 
I will retain one candle 12 inches off, and put two 
candles on the other side 17 inches off. On removing the 
screens you see the index does not move from zero. Now 
the square of 12 is 144, and the square of 17 is 289. 
Twice 144 is 288. The light of these candles, therefore, 
