from Bradky to Fresnel. 101 



which his father, the elder John Bernoulli (b. 1667, d. 1748), 

 had made in 1701 to connect the law of refraction with the 

 mechanical principle of the composition of forces. If two 

 opposed forces whose ratio is ju maintain in equilibrium a 

 particle which is free to move only in a given plane, it follows 

 from the triangle of forces that the directions of the forces must 

 obey the relation 



sin i = fj. sin r, 



where i and r denote the angles made by these directions with 

 the normals to the plane. This is the same equation as that 

 which expresses the law of refraction, and the elder Bernoulli 

 conjectured that a theory of light might be based on it ; but 

 he gave no satisfactory physical reason for the existence of 

 forces along the incident and refracted rays. This defect his 

 son now proceeded to remove. 



All space, according to the younger Bernoulli, is permeated 

 by a fluid aether, containing an immense number of excessively 

 small whirlpools. The elasticity which the aether appears to 

 possess, and in virtue of which it is able to transmit vibrations, 

 is really due to the presence of these whirlpools ; for, owing to 

 -centrifugal force, each whirlpool is continually striving to 

 dilate, and so presses against the neighbouring whirlpools. It 

 will be seen that Bernoulli is a thorough Cartesian in spirit ; 

 not only does he reject action at a distance, but he insists that 

 even the elasticity of his aether shall be explicable in terms of 

 matter and motion. 



This aggregate of small vortices, or " fine-grained turbulent 

 motion," as it came to be called a century and a half later,* is 

 interspersed with solid corpuscles, whose dimensions are small 

 -compared with their distances apart. These are pushed about 

 by the whirlpools whenever the aether is disturbed, but never 

 travel far from their original positions. 



A source of light communicates to its surroundings a 

 disturbance which condenses the nearest whirlpools ; these by 



* Cf . Lord Kelvin's vortex-sponge aether, described later in this work. 



