5-i6 Sir Oliver Lodge on the 



To attempt an answer would involve looking into the 

 manner in which light is retarded by a denser medium, 

 the front being thrown back upon itself (see a detailed 

 theory of refraction by Sir J. J. Thomson, Phil. Mag. 

 Jane 1920, p. 687, and Dec. 1920, p. 715) ; also into the 

 suspected tendency of a wave front to break up laterally 

 and concentrate into quanta-like units akin to those whence 

 it arose. It may suffice for the present to remark that 



the familiar expression for reflected amplitude, — — , is 



represented in the gravitational case by ^ -, or what 



would be written as , or practically - , if, as usual, a is 



written 1+ — . Also that if the momentum conveyed per 



second through any area is equal to the energy per unit 

 length even in a dense medium, — which was Prof. Poynting's 

 assumption and is now being made the subject of careful 

 experimentation by Dr. Barlow, — the momentum of that part 

 which is transmitted increases, because of the longitudinal 

 compression, the increase being accompanied by a pull. 

 The resultant force acting on light when suddenly entering 



.a denser medium perpendicularly comes out 2 . — times 



l-fyu 

 the incident energy-density ; the transmitted energy being 



. -—, of the original, and the transmitted momentum per 



■ 9 \ 2 



second ( ,, \ ; which momentum, instead of remaining 



uniformly distributed, may become concentrated and 

 localised in specks in the immediate neighbourhood of 

 matter or an intense electric field. For the potential close 

 to an electron is one third of a million volts, and the 

 gradient is enormous. The theory of refraction above 

 referred to shows that, inside matter, lateral elements of 

 a wave must contribute to the formation of the new and 

 retarded wave-front ; hence lateral as well as longitudinal 

 concentration is to be expected. 



We may also observe that the density of ordinary sunshine 

 near the earth, being \pl&, is of the order 10 ~ 25 gramme 

 per c.c. ; so if 10 cubic millimetres of earth sunshine, or 

 the equivalent of 1/4600 c.mm. of solar emission, could be 

 checked and condensed till its density was 10 12 it might be 

 converted into an electron of mass 10 ~ 27 gramme. Its 

 original momentum, 3 X 10 ~ 17 , will presumably be communi- 

 cated or restored to whatever stopped its translation ; while 



