[ 944 ] 



XC On the Einstein Spectral Shift. 

 By Sir Oliver Lodge *. 



REFERRING to the interesting paper by Prof. H.J. 

 Priestley on p. 747 o£ last month's Phil. Mag.,. 

 I interpret a sentence in the second paragraph as intending 

 to say that if the Einstein interval ds is transmitted by 

 radiation, instead of the time period dt, then the shift of 

 spectral lines will occur when not the source but the 

 observer is immersed in a strong gravitational field. His 

 further argument is (A) that this transmission of ds is a 

 natural consequence of the principle of equivalence, which 

 makes ds=0 along a ray even in a gravitational field, and 

 (B) that the confirmed gravitational deflexion of a ray can be 

 equally well obtained without depending on the principle of 

 least time, or any other pre-relativity physics, and therefore 

 without admitting the constancy of time period which that 

 principle apparently implies. 



But, I venture to ask, does Prof. Priestley succeed in- 

 establishing proposition B ? 



He says truly that, since ds = along a ray, 



dr 2 + yr 2 de 2 =y 2 dt 2 ..... (7) 



for light ; and he also claims to obtain 



dS 

 ^-r. proportional to y, . . . . (8) 



instead of being merely indeterminate as the ratio of two 

 infinities.; wherefore, combining these equations aud putting 

 l/r=u, he gets 



I -jn ) -i-yu- = constant. 



Whence on differentiation (remembering the variability 

 of 7) 



an equation which may be trusted to behave properly. 

 It is indeed the usual progressing gravitational equation, 

 without the constant term responsible for an orbit. 



* Communicated by the Author. 



