12 PRESSURE OF LIGHT 



Theory of Light flourished, many experiments were 

 made to detect a pressure by allowing light to fall 

 on a disc, like that in fig. I, on a small scale, and 

 very delicately suspended, sometimes in air, some- 

 times in a vacuum. Sometimes the disc was 

 pressed back, sometimes it was drawn forward, 

 &nd no observer obtained conclusive, or even 

 consistent results. 



If these early experimenters had known the 

 Principle of the Conservation of Energy, they 

 would have been able to calculate the value of the 

 pressure, which they were looking for, 1 and it 

 would have worked out on their false theory to 

 double the actual value we now know it to have. 

 Even this double value is far too minute to be 

 detected by the means then attainable. 



Their variable results, now an attraction now a 

 repulsion, were due, no doubt, to two actions which 

 are still the terror of all experimenters on the 

 subject. When they worked in air, the light 

 absorbed by the disc heated it. The disc in turn 

 heated the surrounding air, which expanded and 

 streamed up bodily, forming currents known as 

 "convection currents" simply minute upward 

 gales of wind. If a flat iron plate is heated and 

 then held in front of a lantern these currents 

 form faint shadows on the screen and. may be 



1 Note i, p. 83. 



