306 



SCIENCE 



[N. S. Vol. LI. No. 1317 



action, just as the path of a projectile fired 

 into the air is hent hy the earth's gravitation 

 pull upon, the projectile, the amount of deflec- 

 tion heing in accordance with Newtonian 

 mechanics and Maxwell's electromagnetic 

 theory of light. [If -we assume, as did Xew- 

 ton, that light consists of corpuscles of matter 

 traveling at great velocityj then it is oasy to 

 see why light should be bent under the action 

 of gravity, for a cubic foot of light would 

 in this case differ from a cubic foot of other 

 ponderable material only in matter of weight. 

 Newton in fact, had predicted such bending. 

 But as our knowledge of light advanced we 

 were forced to abandon Newton's theory for 

 the undulatory or wave theory of light — a 

 wave motion in the ether supposed to fill all 

 space, the vibrations being electromagnetic 

 ones according to our latest theory (Max- 

 well's). Light then consisting of some sort 

 of wave motion possesses energy, or the power 

 to do work, and it was furthermore shown 

 about 20 years ago, by a Russian physicist, 

 Lebedew, and by two American physicists, 

 Nichols and Hull, that light exerts a meas- 

 urable pressure when it falls upon a surface 

 just as would material particles when fired at 

 that surface. That light exerts pressure was 

 in fact predicted by Maxwell a half century 

 ago, but it was an open question whether light 

 also had iveighi. The pressure of light re- 

 sulted from the electromagnetic energy in- 

 herent in light, by which it is endowed with 

 inertia just as is a body of material mass. 

 Woiild gravity act upon something having 

 electromagnetic inertia in the same way as 

 upon a body of material mass? If so, the 

 precise gravitational effect upon light could 

 be predicted.] If a ray of light from a dis- 

 tant star just grazed the sun's edge (limb), 

 it would be bent inwards (towards the sun) 

 by 0".44, as viewed by a solar observer. As 

 the ray of light passed out of the sun's gravi- 

 tational field on its journey to the earth it 

 would suffer another deflection of about 0".44, 

 and in such a way that the final and total 

 bending as perceived by an observer on the 

 earth, would be away from the sun 0".8Y — 

 the angle which an object one inch high would 



subtend at a distance of three and three fourth 

 miles. 



(6) Einstein Effect. — Twice the deflection 

 of the rays of light predicted in (a), this time 

 again hy the sun's gravitational action, hut 

 according to the principles of Einstein's gen- 

 eralized relativity theory. (These principles 

 are tersely stated by Professor A. G. "Web- 

 ster) :i^ 



First, that of the constancy of the velocity of 

 light with respect to all directions and to any sys- 

 tem moving with any velocity whatever with re- 

 spect to any other system; second, a relation be- 

 tween time and distance such tihat either of two 

 bo'dies seem shortened in the direction of their 

 relative motion by an observer attached to the 

 other; third, that it is impossible to distinguish a 

 gravitational field from the acceleration of the 

 frame of reference; and fourth, tha,t everything 

 that has mass, as determined by inertia, has masi 

 of the sort determined by weight or attractability. 



According to the Einstein law of gravita- 

 tion, the deflection of a ray of light which 

 grazed the sun's limb would be away from the 

 sun by 1".74^^, as we, or anyone outside the 

 sun's gravitational field, might perceive it. 



(c) Refraction in the Solar Atmosphere. — 

 Bending of rays of light by refraction in 

 passing through the sun's atmosphere, which, 

 in more or less attenuated form, is known to 

 extend out so far that the rays from all the 

 stars concerned in the measurements would 

 have to pass through it on their way to the 

 earth.^^ [Such bending of light actually 

 takes place all the time as the rays from the 

 sun and other celestial bodies pass through our 

 own atmosphere; the amount of atmospheric 



" The Beview, January 31, 1920, p. 116. 



12 See A. S. Eddington's "Report on the Rela- 

 tivity Theory of Gravitation," Ijondon, 1920, p. 55. 



13 See Dr. H. F. Newall's suggestive note in 

 Monthly Notices of the Boyal Astronomical So- 

 ciety, Vol. LXXX., No. 1, November, 1919. Mr. 

 Jonek-heere (Tlie Observatory for August, 1919, 

 Vol. XLI., p. 216) suggested that refractions may 

 be caused by " a hypothetical condensation of ether 

 near the sun." This hypothesis is treated by L. 

 Silberstein in connection with the theory of Stokes- 

 Planck's ether in the Phil. Mag., Vol. 39, pp. 161- 

 170, February, 1920. 



