344 Mechanical Action of Light. [April, 
the table. The electric light falling vertically downwards on 
it, and much of the power being cut off by water and alum 
screens, the rotation is slow. I bring a candle near and 
the speed increases. I now lift the radiometer up, and place 
it lull in the eledtric light, projecting its image direct on 
the screen, and it goes so rapidly that if I had not cut out 
the four pieces of pith of different shapes you would have 
been unable to follow the movement. 
The speed with which a sensitive radiometer will revolve 
in the sun is almost incredible ; and the eledtric light such 
as I have it in this lantern cannot be far short of full sun- 
shine. Here is the most sensitive instrument I have yet 
made, and I project its image on the screen, letting the full 
blaze of the eleCtric light shine upon it. Nothing is seen 
but an undefined nebulous ring, which becomes at times 
almost invisible. The number of revolutions per second 
cannot be counted, but they must be several hundreds, for one 
candle has made it spin round forty times a second. 
I have called the instrument the radiometer because it 
will enable me to measure the intensity of radiation falling 
on it by counting the revolutions in a given time ; the law 
being that the rapidity of revolution is inversely as the square 
of the distance between the light and the instrument. 
When exposed to different numbers of candles at the same 
distance off, the speed of revolution in a given time is in 
proportion to the number of candles ; two candles giving 
twice the rapidity of one candle, and three, three times, &c. 
The position of the light in the horizontal plane of the 
instrument is of no consequence, provided the distance is not 
altered; thus two candles, one foot off, give the same num- 
ber of revolutions per second, whether they are side by side 
or opposite to each other. From this it follows that if the 
radiometer is brought into a uniformly lighted space it will 
continue to revolve. 
It is easy to get rotation in a radiometer without having 
the surfaces of the discs differently coloured. Here is one 
having the pith discs blacked on both sides. I project its 
image on the screen, and there is no movement. I bring 
a candle near it, and shade the light from one side, when 
rapid rotation is produced, which is at once altered in direc- 
tion by moving the shade to the other side. 
I have arranged here a radiometer so that it can be made 
to move by a very faint light, and at the same time its rota- 
tion is easily followed by all present. In this bulb is a 
large six-armed radiometer carrying a mirror in its centre. 
The mirror is almost horizontal, but not quite so, and there- 
