1918,] On the Pressure of Light. 431 
the lamp consumed a current 6-6 amps. under a pressure of 220 
volts. The amount of energy passing through the lens and 
focussed on the vanes is therefore given by 
220 x 6°6 x 10" 
4x (d)” 
The whole pressure on the silvered surface is therefore 
a (3°25)” ergs per sec. 
a (l= 8 (ey 
Cc 
where ¢ velocity of light and ¢«=fraction of energy absorbed 
by and reflected from glass surfaces (lens and containing vessels) 
and p= fraction of energy reflected from the silvered face. 
RESULTS OF OBSERVATIONS. 
election remains quite steady and follows very faithfully the 
fluctuations of the arc. When the positive pole was focussed the 
deflection observed was generally 3 to 4 times the deflection for 
the negative pole. When all the precautions above mentioned 
Wete taken, the deflection was found to be always in the right 
7éction. When the filament lamp was used as the source of 
light, all irregularities due to the variation of the source of light 
Vanished. As soon as light is struck, the spot of light slowly 
«eps up towards the new position of equilibrium about whic 
ut oscillates in accordance with the equation (i). © 
_ Ultimately the oscillation dies away and the spot becomes 
dite steady, which could be maintained for 15 minutes (we 
did not try ‘to keep the spot steady for a greater length of time 
. suse the tungsten filaments, being kept in a horizontal posi- 
the bap sredually deformed on account of their plasticity at 
© high temperature within the lamp. 
Surface was approximately one half of that of the 
silvered one. If the wet of light was very intense so muc 
Was absorbed that the junctions (which were all of shellac) 
- sib] od’ off. antitative experiments were therefore impos- 
® with that surface 
below —— the results of our quantitative experiments is given 
