ASTRO-PHYSICS 293 



possessed by disconnected systems flying through 

 space from distant sources. On the other hand, 

 the opposite velocities, constant in amount, show 

 that the two stars cannot be two members of 

 the same group, colliding with each other as an 

 effect of ill-directed mutual gravitation, which 

 would lead to a decrease in velocity as the stars, 

 after collision, receded from each other. The 

 theory of collision has perforce been abandoned. 

 No satisfactory hypothesis has yet been proposed 

 in its place. Perhaps the one least open to 

 objection is that which regards the luminosity 

 as due to the passage of a star, possibly a dark 

 one sometimes double, through the scattered 

 matter constituting a nebula, in much the same 

 way as a shooting star shines only during its 

 transit through the earth's atmosphere. 



Many years ago Clerk Maxwell showed 

 theoretically that a stream of light, incident on 

 a body, should produce a pressure in the direction 

 of the advancing rays. Maxwell deduced the 

 effect from the electro-magnetic theory of light, 

 but it has since been shown by Larmor to be 

 necessary on almost any wave theory. The 

 undulations must possess energy, and, therefore, 

 momentum. An absorbing body is gaining 

 momentum, and therefore experiences a pressure 

 in the direction of the incident beam. A reflect- 

 ing body reflects the same momentum back again, 

 and therefore is acted on by a double pressure. 

 This result was first confirmed experimentally by 

 Professor Lebedef, of Moscow. The difficulties 

 to be overcome are best appreciated by the 

 statement that when bright sunlight falls on a 



