-412 The Theory of Aether and Electrons in the 



when transmitting light : the orientation of the wave-fronts of 

 the light will consequently in general be altered ; and the direc- 

 tion in which a heavenly body is seen, being normal to the wave- 

 fronts will thereby be affected. But if the aethereal motion 

 is irrotational, so that the elements of the aether do not 

 rotate, it is easily seen that the direction of propagation of the 

 light in space is unaffected ; the luminous disturbance is still 

 propagated in straight lines from the star, while the normal 

 to the wave-front at any point deviates from this line of 

 propagation by the small angle ujc, where u denotes the 

 component of the aethereal velocity at the point, resolved at 

 right angles to the line of propagation, and c denotes the 

 velocity of light. If it be supposed that the aether near the 

 earth is at rest relatively to the earth's surface, the star will 

 appear to be displaced towards the direction in which the 

 earth is moving, through an angle measured by the ratio of 

 the velocity of the earth to the velocity of light, multiplied by 

 the sine of the angle between the direction of the earth's 

 motion and the line joining the earth and star. This is 

 precisely the law of aberration. 



An objection to Stokes's theory has been pointed out by 

 several writers, amongst others by H. A. Lorentz.* This is, 

 that the irrotational motion of an incompressible fluid is 

 completely determinate when the normal component of the 

 velocity at its boundary is given : so that if the aether were 

 supposed to have the same normal component of velocity as 

 the earth, it would not have the same tangential component of 

 velocity. It follows that no motion will in general exist which 

 satisfies Stokes's conditions ; and the difficulty is not solved in 

 any very satisfactory fashion by either of the suggestions which, 

 have been proposed to meet it. One of these is to suppose that 

 the moving earth does generate a rotational disturbance, which, 

 however, being radiated away with the velocity of light, does not 

 affect the steadier irrotational motion ; the other, which was 



* Archives Neerl, xxi (1896), p. 103. 



