i875-J 
Mechanical A ction of Light. 
245 
fore when I throw a beam of electric light vertically down- 
wards on to the central mirror, the light is reflected off at a 
slight angle, and as the instrument rotates its movement is 
shown by the spot of light travelling round the ceiling in a 
circle. Here again the fog helps us, for it gives us an im- 
ponderable beam of light moving round the room like a solid 
body, and saving you the trouble of looking up to the ceiling. 
I now set the radiometer moving round by the light of a 
candle, and I want to show you that coloured light does not 
very much interfere with the movement. I place yellow 
glass in front, and the movement is scarcely diminished at 
all. Very deep coloured glass, you see, diminishes it a little 
more. Blue and green glass make it go a little slower, but 
still do not diminish the speed one-half. I now place a screen 
of water in front : the instrument moves with diminished 
velocity, rotating with about one-fourth its original speed. 
Taking the aCtion produced by a candle flame as 
100 
Yellow glass reduces it to ... 
89 
Red ,, 
,, ,, ... 
7i 
Blue ,, 
,, ,, ... 
56 
Green ,, 
,, ,, . ® 0 
56 
Water 
,, ,, ... 
26 
Alum 
,, ,, ... 
15 
I now move the candle a little distance off, so as to make 
the instrument move slower, and bring a flask of boiling water 
close to it. See what happens. The luminous index no longer 
moves steadily, but in jerks. Each disc appears to come up 
to the boiling water with difficulty, and to hurry past it. 
More and more sluggishly do they move past, until now one 
has failed to get by, and the luminous beam, after oscillating 
to and fro a few times, comes to rest. I now gradually bring 
the candle near. The index shows no movement. Nearer 
still. There is now a commencement of motion, as if the 
radiometer was trying to push past the resistance offered 
by the hot water; but it is not until I have brought the 
candle to within a few inches of the glass globe that rotation 
is recommenced. On these pith radiometers the aCtion of 
dark heat is to repel the black and white surfaces almost 
equally, and this repulsion is so energetic as to overcome the 
rotation caused by the candle, and to stop the instrument. 
With a radiometer constructed of a good conductor of 
heat, such as metal, the aCtion of dark heat is different. 
Here is one made of silvered copper, polished on one side 
and lampblacked on the other. I have set it moving with a 
candle slightly the normal way. Here is a glass shade heated 
