602 



SCIENCE 



[N. S. Vol. XXVI. No. 670 



time will not only be devoted to teaching but 

 also to the advancement of research in trop- 

 ical medicine. Paul C. Freer, 

 Dean of the Philippine Medical School 



SOME ASTRONOMICAL CONSEQUENCES OF 

 THE PRESSURE OF LIGHT ^ 



The experiments of Lebedew and Nichols 

 and Hull have proved conclusively that light 

 presses against any surface upon which it 

 falls, and the extraordinarily accurate experi- 

 ments of Nichols and Hull have fully con- 

 firmed Maxwell's calculation that the pressure 

 per square centimeter is equal to the energy 

 in the beam per cubic centimeter. 



A clearer idea of the efiect of light or radia- 

 tion pressure is obtained by thinking of a 

 beam of light as a carrier of momentum. We 

 then see that not only does it press against a 

 receiving surface, but also against the surface 

 from which it started. 



Some experiments by Dr. Barlow and my- 

 self appear to bring to the front this concep- 

 tion of light as a momentum carrier. If a 

 beam falls on a black surface at an angle to 

 the normal, there should be a tangential stress 

 along the surface. An experiment was de- 

 scribed in which light fell on a blackened disc 

 at the end of a torsion arm, the disc being at 

 right angles to the arm.^ The disc was pushed 

 round by the tangential stress. The experi- 

 ment was carried out in a partially exhausted 

 vessel, but the residual air was a source of 

 disturbance by convection and radiometer ef- 

 fects. A better experiment was made by sus- 

 pending a disc of mica blackened beneath, 

 about two inches in diameter by a quartz fiber, 

 the disc being horizontal and suspended from 

 its center. When a beam of light fell at 45° 

 on a parj; of the disc, the horizontal component 

 of the beam being at right angles to the radius 

 to the part where it fell, the disc moved round 

 through the combined effects of convection, 

 radiometer action and the tangential stress, 

 When the beam was allowed to fall on the 

 same place at 45° on the other side of the 

 vertical, convection and radiometer action 



' Abstract of an address before the Royal In- 

 stitution of Great Britain. 



= PAa. Mag., IX. (1905), p. 169. 



were very nearly as before, but the tangential 

 stress was reversed. The difEerence in torsion 

 in the two cases was twice that due to the 

 tangential stress. An experiment with prisms" 

 was also described. 



Regarding a beam of light as a momen- 

 tum carrier, it is easily seen that if the re- 

 ceiving surface has velocity u towards the 

 source and the velocity of light is U, the pres- 

 sure is increased by the motion by the fraction 

 u/U. If the velocity is reversed, the pressure 

 is decreased by this fraction. This is the 

 " Doppler reception eilect." 



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 pressure on the 

 source is Z7/(J7zpw) of its value when the 

 source is at rest. This is the " Doppler emis- 

 sion effect." 



In considering the consequence of light 

 pressure, it is necessaiy to know the tempera- 

 ture 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 tempera- 

 ture of the sun is about twenty times as high, 

 say 6000° Abs. The temperature of the par- 

 ticle 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 di- 

 ameter 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 dim- 

 inished, the ratio of gravitation pull to lighl 

 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 one inch. 

 The consequence of this on a swarm of m^e- 



'Ibid., p. 404. 



