€54 Dr. Gr. Green on a I 1 luid 



field, such as that found in the immediate neighbourhood of 

 the sun, at the total Solar eclipse of May 29, 1919. 



Another important suggestion contained in the analogy 

 between the longer waves in shallow water and luminous and 

 electromagnetic waves in the aether is — that the velocity o£ 

 light in aether is according to this analogy to be regarded 

 as a minimum wave-velocity. Just as the minimum wave- 

 velocity in water can be determined from equations (1) and 

 (2) above, as a determinate function of g, Jt, and T, so the 

 analogy leads us to expect to find some definite relation 

 between the velocity of light, the elastic constant or the 

 density of the aether and the constant of gravitation. The 

 view that the velocity of light in aether is to be regarded as 

 a minimum is of importance also later on when we come 

 to consider the phenomena accompanying the motion of 

 particles through the aether at speeds approaching the 

 velocity of light in their relation to analogous phenomena 

 in ordinary fluids. 



The similarity existing between the two media, shallow 

 water and aether, with respect to the propagation of waves 

 is not confined to general relations between wave-length 

 and speed of propagation. It can readily be verified tor 

 example that the ordinary laws of reflexion and refraction 

 are obeyed in the case of shallow-water waves travelling in 

 fluid which is otherwise at rest. In addition, it is clear that 

 all phenomena involving the Doppler effect would obtain in 

 ordinary fluids. By suitable suppositions regarding the 

 relations between matter and aether it is also possible to 

 extend the analogy so as to include a representation of the 

 refraction and dispersion of aether waves in passing through 

 material bodies. Inasmuch as a sudden increase in the 

 depth oil the fluid would involve an alteration in the velocity 

 of waves in such a w T ay as to cause this to vary according to 

 the wave-length — a region of greatly increased depth might 

 be regarded as corresponding in the analogy to a dispersive 

 medium. It would be impossible, however, to represent the 

 various forms of dispersive media by means of a variation 

 in the depth of the fluid alone. A much more complete 

 representation of dispersion and refraction of luminous 

 waves <>n passing from aether into matter can be obtained 

 by assuming the matter to be represented by a portion 

 of space permeable by the fluid containing a very large 

 number of vibrators having periods of their 'own. In 

 the number and values of the natural periods of the 

 vibrators, and in variations of their density, and in the 

 closeness or openness of their arrangement within the fluid, 



