102 The Luminiferous Medium i 



their condensation displace the contiguous corpuscles from their 

 equilibrium position ; and these in turn produce condensations 

 in the whirlpools next beyond them, so that vibrations are 

 propagated in every direction from the luminous point. It is 

 curious that Bernoulli speaks of these vibrations as longitudinal, 

 and actually contrasts them with those of a stretched cord, 

 which, " when it is slightly displaced from its rectilinear form, 

 and then let go, performs transverse vibrations in a direction at 

 right angles to the direction of the cord." When it is 

 remembered that the objection to longitudinal vibrations, on 

 the score of polarization, had already been clearly stated by 

 Newton, and that Bernoulli's aether closely resembles that 

 which Maxwell invented in 1861-2 for the express purpose of 

 securing transversality of vibration, one feels that perhaps no 

 man ever so narrowly missed a great discovery. 



Bernoulli explained refraction by combining these ideas 

 with those of his father. Within the pores of ponderable 

 bodies the whirlpools are compressed, so the centrifugal force 

 must vary in intensity from one medium to another. Thus a 

 corpuscle situated in the interface between two media is acted 

 on by a greater elastic force from one medium than from the 

 other; and by applying the triangle of forces to find the- 

 conditions of its equilibrium, the law of Snell and Descartes 



r may be obtained. 



Not long after this, the echoes of the old controversy 



' between Descartes and Fermat about the law of refraction 

 were awakened* by Pierre Louis Moreau deMaupertuis (b. 1698,, 

 d. 1759). 



It will be remembered that according to Descartes the 

 velocity of light is greatest in dense media, while according to- 

 Fermat the propagation is swiftest in free aether. The argu- 

 ments of the corpuscular theory convinced Maupertuis that on 

 this particular point Descartes was in the right ; but never- 

 theless he wished to retain for science the beautiful method by 

 which Fermat had derived his result. This he now proposed 



*Mem. de 1'Acad., 1744, p. 417. 



