1918.| On the Pressure of Light. 427 
directly proportional to the difference of pressure on the two 
sides, and the pressure of gas within the vessel. Lebedew 
uced the pressure to about 1/20000th of a m.m. and was 
almost able to eliminate the radiometer action, and verify the 
Pressure qualitatively to about 20°, of the theoretical pressure. 
€ early experiments of Nichols and Hull were under- 
taken in order to investigate the different disturbing effects in 
the apparatus of Zéllner. They found that the total disturbing 
“Hect is the resultant of the following :— 
(i) the radiometer action—due to the unequal heating of 
the two sides of the vane: 
(li) convection currents—due to the rush of air towards 
rts warmed by the passage of the pencil of 
f the vanes when thes 
Oe Sta Aad: 
(iii) a rocket action—due to the escape of particles of gas 
from the surface o je «are 
heated by the incident light. 
es e 
silvered ; for further information on the point 
. i lly > Hu ] 
the dist oat 
brite i i act wit e 
Ptticaiepe of a thin cover-glass was placed in contact wi 
