230 Mechanical Action of Light. [April, 
and it is attracted, and follows the rod round as a magnetic 
needle follows a piece of iron. 
The repulsion by radiation is the key-note of these re- 
searches. The movement of a small bar of pith is not very 
distinct, except to those near, and I wish to make this repul- 
sion evident to all. I have therefore arranged a piece of 
apparatus by which it can be seen by all present. I will, by 
means of the electric light, project an image of a pendulum 
suspended in vacuo on the screen. You see that the ap- 
proach of a candle gives the bob a veritable push, and, by 
alternately obscuring and uncovering the light, I can make 
the pendulum beat time to my movements. 
FIG. I . 
What then is the cause of the contradictory adtion in these 
two bulbs — attraction in one, and repulsion in the other? 
It can be explained in a few words. Attraction takes place 
when air is present, and repulsion when air is absent. 
Neutrality, or no movement, is produced when the vacuum 
is insufficient. A minute trace of air in the apparatus inter- 
feres most materially with the repulsion, and for a long time 
I was unaware of the powerful action produced by radiation 
in a “ perfect ” vacuum. 
