324 Sir Joseph Larmor 



Gravitation and Light. By Sir Joseph Larmor, St Johirs 

 College, Lucasian Professor. 



[Read 26 January 1920.] 



1. Newton's provisional thoughts on the deep questions of 

 physical science were printed at the end of the second edition 

 of the Opticks in 1717. As he explains in the Preface " . . .at the 

 end of the Third Book I have added some questions. And to shew 

 that I do not take Gravity for an Essential Property of Bodies, 

 I have added one Question concerning its Cause, chusing rather to 

 preface it by way of a Question, because I am not yet satisfied 

 about it for want of Experiments." In the first and next following 

 Queries he gives formal expression to the idea that "Bodies Act 

 upon Light at a distance and by their action bend its Rays. ..." 



What was thus propounded in general terms as an explanation 

 of the diffraction of light in passing close to the edge of an obstacle, 

 assumed a more definite but different form in the hands of the 

 physically-minded John Michell*; in Phil. Trans. 1767 he insisted 

 that the Newtonian corpuscles of light must be subject to gravita- 

 tion like other bodies, therefore that the velocities of the corpuscles 

 shot out from one of the more massive stars vrould be sensibly 

 diminished by the backward pull of its gravitation, and thus that 

 they would be deviated more than usual by a glass prism, a supposi- 

 tion which he proposed to test by experiment. He also speculated 

 that the scintillation of the stars might be due to the small number 

 of corpuscles which reach the eye from a star, amounting perhaps 

 to only a few per second. 



The forces, of molecular range, that would have to be con- 

 cerned, on the lines of Newton's Query, in the diffraction of light 

 would be of course enormously more intense than gravitation : but 

 the other Newton-Michell theory of the gravitation of light rays 

 is paralleled in both its aspects with curious closeness in certain 

 modern physical speculations. 



It will be observed that this notion of light being subject to 

 gravitation makes its velocity exceed the limiting velocity c, which 

 on electrodynamic theory could not be attained by any material 

 body. But there need not be a discrepancy there: for the limit 

 arises because a material body is supposed to acquire more and 

 more inertia, belonging to energy of its motion, without limit as 

 its velocity increases, whereas the quantum of energy in the hypo- 

 thetical light-bundle presumably would remain sensibly the same — - 

 at any rate we would be free to make hypotheses in absence of 

 any knowledge. 



* See Memoir of John Michell (of Queens' College), by Sir A. Geikie, Cambridge 

 Press, 1918. 



