354 
ME. W. CROOKES ON EEPULSION EESITLTINGr EEOM EADIATION. 
sun, or with the electric light shining on it, the movement is very striking, as it shows 
four disks of light chasing each other round the room. 
183. A radiometer was made of the following construction (fig. 11). a is the disk of 
pith, black on one side ; it is attached to a thin brass arm revolving on a -p io , -y 
needle-point ; b is a mirror, seen in section, hanging from the other side 
of the brass arm, and having its plane perpendicular to the plane of the 
pith disk a. The needle-point works in a jewel cup c, and is prolonged 
upwards into the tube d , which is sealed into the bulb, and in which the 
needle fits loosely. Behind the mirror b is a very small magnet, to give 
direction to the arm. The object of having the upper tube ( d ) is to 
prevent the arm from coming off in carriage : with the four, or more, 
armed radiometers it is easy to get the movable part on when it falls off, 
but with this one-armed instrument it would be an almost impossible 
feat. (This artifice of an upper protecting tube is one I have had occasion 
to adopt on many occasions, and I find it very convenient.) The moving V 
part weighs 2'42 grains ; it revolves somewhat slowly when a candle is brought near, 
owing to the interference of the magnet. When the magnet is rendered nearly astatic 
by another magnet near it, and an index ray of light is reflected from the mirror, this 
radiometer is sensitive to a candle several yards off. It is a more convenient instrument 
for measuring different kinds of radiation than is the one on a similar principle described 
in par. 135, but, owing to the friction on the needle-point, it is not so sensitive. 
184. A large radiometer in a 4-inch bulb was made with ten arms, eight of them 
being of brass, and the other two being a long watch-spring magnet. The disks are 
of pith, blackened on one side. The weight of u the fly is 1T87 grains. This moves 
very rapidly for so heavy an instrument. The power of the earth on the magnet 
is too great to allow the arms to be set in rotation, unless a candle is brought very near ; 
but once started it will continue to revolve with the light some distance off. This was 
made to enable me to communicate motion from the interior of the bulb to the outside. 
By suspending a magnet near the bulb, it oscillates to and fro with every revolution of 
the radiometer. The movement can thus either be projected on a screen or it may work 
a telegraphic instrument, and thus give a visible demonstration or a permanent record 
of the revolutions caused by any source of light under examination. As a self-registering 
photometric instrument this form of radiometer would be of considerable value. 
185. A large six-disk radiometer was made in a 4-inch bulb. Immediately over the 
needle support a silvered glass mirror was fixed almost, but not quite, horizontal. By 
throwing a beam of light vertically downwards on this mirror it is reflected upwards 
at a slight angle, and as the radiometer revolves the movement can be seen by an audience 
as a spot of light traversing in a circle around the ceiling. The effect of various lights, 
coloured screens, &c. in modifying the rapidity of movement can be well illustrated in 
this manner. 
In a subsequent paper, which I hope soon to have the honour of laying before the 
Society, I propose to give the results of my experiments on the different rays of the 
