HOW LIGHT EXERTS PRESSURE 



seen rising like smoke. It depends entirely on 

 the lie of the plate whether these rising streams of 

 air will tend to press the plate back or to draw it 

 forward. The action of the air currents on a disc 

 heated by a beam of light may easily be many 

 times greater than the pressure of 

 the light. 



When they worked in a vacuum 

 probably another action came into 

 play, an action discovered and 

 investigated by Sir William 

 Crookes, who invented a beautiful 

 little instrument to show it, which 

 he named the Radiometer. The 

 radiometer in its commonest form 

 consists of four small mica discs 

 fixed at the four ends of a hori- 

 zontal cross as in fig. 3. The cross 

 is free to spin round on a pivot as 

 frictionless as possible, and it is 

 contained in a highly exhausted 

 bulb about three inches in dia- 

 meter. Each of the vanes is blackened on one side, 

 and when a lighted match or candle is brought 

 near the bulb, the blackened faces retreat from the 

 source of light while the unblackened faces move 

 towards it. At first it was supposed that the action 

 might be directly due to the pressure of light, but 

 it is easy to see that this pressure would produce 



FIG. 3. 



