242 
Mechanical Action of Light. 
[April, 
in the same direction half round. This adtion will be again 
repeated, the differential adtion of the light on the black and 
white surfaces keeps the bar moving, and the result will be 
rotation. 
Here is such a pith bar, blacked on alternate sides, and 
suspended in an exhausted glass bulb (Fig. 7). I projedt its 
FIG. 7. 
A 
image on the screen, and the strong light which shines on 
it sets it rotating with considerable velocity. Now it is 
slackening speed, and now it has stopped altogether. The 
bar is supported on a fibre of silk, which has twisted round 
till the rotation is stopped by the accumulated torsion. I 
put a water screen between the bar and the eledtric light to 
cut off some of the adtiverays, and the silk untwists, turning 
