RADIATION OF ENERGY. 357 



the second place it gives to the quantity, c, which we call the velocity of light 

 certain very peculiar properties. 



If the principle of relativity be adopted as a working hypothesis we ha\ 



ree 



alternatives as to the nature of radiant energy: it must exist independent Ii 

 in space; it must be carried by small particles of matter, after the manner of the 

 corpuscular theory of light; or else it has no real existence in space. I think I 

 have shown by my discussion of the nature of energy and of ! mst in's light 

 quantum theory, in the next section, that the first alternative is untenable. 

 The phenomena of the dispersion, interference and polarization of light appear to 

 preclude the hypothesis of its transmission by small particles, and we an' there- 

 fore driven to the last alternative that energy has no real existence in space. 



If c is a universal constant, as is asusmed by the relativity theory, and 

 measurements of it will always have the same value whatever the motion of th< 

 observer, it cannot be of the nature of a mechanical velocity. This I think i| 

 also indicated by the fact that it is equal to the ratio of the electrostatic to the 

 electromagnetic unit of quantity. 



If the second postulate of relativity is true it appears to me that the reciprocal 

 of the so-called velocity of light must be of the nature of a time reaction constant 

 That is, a light disturbance originating at one point in space becomes evident at 

 another point at a distance s at a time t given by 



t = ks 



where k = 1/c. The reaction time, t, according to this view is proportional 

 to s. Since nearly all actions and reactions of matter, including thermal, chem- 

 ical, electrical, and possibly gravitational act ions require a certain time to act 

 even if the distances by which they are separated are very small, this view of the 

 reciprocal of c does not seem to me to be very unreasonable. 



Since the second postulate of relativity says that the apparent velocity of 

 light is independent of the motion of the source and of the observer, and the 

 first postulate says that any motion between the source and the observer is 

 relative, the first postulate resolves itself into the statement that the velocity of 

 light is independent of the velocity of the source. This is the form in which 



instein originally gave it. He called it the principle of the constancy of light 

 velocity. 



If the velocity of light did depend on the velocity of the source, and the two 

 velocities were additive, it would explain all the phenomena which the principle 

 of relativity explains, but there is no evidence that the velocity of light does 

 depend upon the velocity of the source. No decisive evidence appears to be at 

 hand at present to determine this point, but what evidence we do have seems to 

 show that the velocity of light is independent of the source. 1 



1 Tolman, The Second Postulate of Relativity, Physical Review, 31, 26-W, 1910; Whittaker, History 

 of the Theories of the iEther, pp. 413-415; Lorentz, Physikalische Zeitschrift, 11, pp. 1239, 1240, 1910. 



