1875.] The Mechanical Action of Light. 347 
point resting in a jewel cup, sothat the arms are able to 
revolve horizontally upon the centre pivot, in the same 
manner as the arms of Dr. Robinson’s anemometer revolve. 
To the extremity of each arm is fastened a thin disc of pith, 
white on one side and lamp-blacked on the other, the 
black surfaces of all the discs facing the same way. The 
whole is enclosed in a thin glass globe, which is then 
Fics. 7 and 8. 
a. A very fine needle point. 
b. Two pieces of straw. 
c. Jewel cup. 
dddd. Four pith discs, blackened on one side. The arms between the straw in the 
centre and the discs are bent glass fibres. 
e. Glass support holding cup. 
f. Cement to keep the support ¢ in its place, 
