26 president's address — section a. 



This puts absolute motion and motion relative to the ether in the 

 same category, both as physically unrecognisable. Einstein 

 accepts the results of such experiments as Michelson and Morley's, 

 Rayleigh's, Brace's, Trouton and Noble's, as exhibiting an em- 

 pirical principle — the principle of relativity, according to which 

 it is impossible for an observer to detect his absolute motion 

 relative to a stationary ether. Setting out with that assumption 

 he develops a new system of dynamics, and lays down how length 

 and time are to be measured in a moving system so that after 

 they have been so measured no positive effects are to be expected 

 from a convection of optical, electrical, or other apparatus in 

 which the observer shares. It has been pointed out by Bumstead 

 that if the principle of relativity is accepted it will occupy a position 

 analagous to that of the second law of thermodynamics. It rests 

 on a similar basis, in that no deviations from it have been observed. 

 Indeed the analogy may be made more complete, for Larmor^ has 

 shown that the denial of the principle leads to a kind of perpetual 

 motion, by which the kinetic energy of any body might be ex- 

 hausted and the body be brought to rest with reference to the 

 ether. There is, however, an enormous difference in the breadth 

 of the evidence on which the two principles rest. Violations of the 

 principle of relativity lead only to minute effects, which must be 

 sought in difficult and recondite experiments. 



H. A. Lorentz's treatment^ of most of the problems of 

 moving bodies, based on the fundamental equations of the electro- 

 magnetic field, has led up to the present to the same result as 

 Einstein's principle. Lorentz's treatment makes the ether sub- 

 stantial and important ; the other decreases the ether's import- 

 ance. The principle of relativity, there can be no doubt, formu- 

 lates our knowledge more clearly. 



Physical Phenomena in Moving Bodies — First Order Positive 

 Ejects. — How some of the experimental results fit in with the 

 theories mentioned above, and to what degree inconsistencies have 

 been removed is considered below. 



The aberration of light, as discovered by Bradley, in 1728, is 

 usually explained on what is really a projectile theory of light. 

 It is often pointed out to illustrate the theory that an observer 

 looking through the hole made by a projectile from a stationary 

 gun in a moving ship would not see the gun's position, but a position 

 ahead of it. This makes the angle of aberration of light, «, equal 

 to the ratio of the velocity of the earth, u, to velocity of light in 

 the telescope, v, i.e., a^zufv, when « is small. 



Airy's experiment on the aberration of light, in which the 

 telescope was filled with water in order to diminish the velocity of 

 the light in the telescope, but with no effect on the angle of aberra- 

 tion, is a more general case than Bradley's. The emission 



1. "Aether and Matter." 



2. " The Theory of Electrons." 



