340 Professor J. H. PoynUng \_May 11, 



Regarding a beam of light as a momentum carrier, it is easily- 

 seen that if the receiving surface has velocity u towards the source 

 and the velocity of light is U, the pressure is increased by the 



motion by the fraction ^. If the velocity is reversed, the pressure 



is decreased by this fraction. This is the " Doppler reception effect." 



If the source is moving, and we assume that the amplitude of the 



emitted waves depends on the temperature and nature of the source 



alone, it can be shown that the pressui'e on the source is ^ 



U + w 



of its value when the source is at rest. This is the " Doppler emission 



effect." 



In considering the consequence of light pressure, it is necessary 

 to know the temperature of a body exposed to the sun's radiation. 

 It can be shown that a small black particle, at the distance of the 

 earth from the sun, has about the mean temperature of the earth's 

 surface, say 300° Abs., and that the temperature of the sun is about 

 twenty times as high, say 6000° Abs. The temperature of the particle 

 varies inversely as the square root of its distance from the sun. 



The direct pressure of sunlight is virtually a lessening of the 

 sun's gravitation pull. On bodies of large size this is negligible. 

 On the earth it is only about a forty-billionth of the sun's pull, but 

 the ratio increases as the diameter decreases, and a particle one forty- 

 billionth of the earth's diameter, and of the same density, would be 

 pushed back as much as it is pulled in, if the law held good down to 

 such a size. If the radiating body is diminished, the ratio of gravi- 

 tation pull to light push is similarly diminished, and it can be shown 

 that two bodies of the temperature of the earth's surface and of the 

 earth's mean density would neither attract nor repel each other, if 

 their diameter was about 1 inch. The consequence of this on a swarm 

 of meteorites is obvious. It is probable that this balancing of 

 gravitation and light pressure must be taken into account in the 

 motion of the particles supposed to constitute Saturn's rings. 



When we consider the motion of a small particle round the sun, 

 we have, first, the direct pressure lessening gravitation. If it has 

 density equal to that of the earth and diameter -j-gVo" inch, the lessened 

 pull at the distance of the earth will imply a lengthening of the year 

 by nearly two days. Secondly, the Doppler emission effect comes 

 into play, for the particle crowds forward on its own waves emitted 

 in front, and draws away from those emitted behind, so that there 

 is increase of pressure in front and a decrease behind. Thus there 

 is a force resisting the motion. The particle will then tend to fall 

 inwards in its orbit, and in the case considered about S(»0 miles in 

 the first year. It would probably move in a spiral into the sun, and 

 reach it in less than 100,000 years. A particle 1 inch in diameter 

 would reach the sun from the earth in less than a hundred million 

 years. 



The Doppler reception effect will not come into play in a circular 



