122 The Lumini/erous Medium, 



undulations of sound, consisting simply in the direct and retro- 

 grade motions of the particles in the direction of the radius,, 

 with their concomitant condensation and rarefactions. And 

 yet it is possible to explain in this theory a transverse vibration,, 

 propagated also in the direction of the radius, and with equal 

 velocity, the motions of the particles being in a certain constant 

 direction with respect to that radius ; and this is a polarization" 



In an article on " Chromatics," which was written in 

 September of the same year* for the supplement to the 

 Encyclopaedia Britannica, he says :f " If we assume as a mathe- 

 matical postulate, on the undulating theory, without attempting 

 to demonstrate its physical foundation, that a transverse motion 

 may be propagated in a direct line, we may derive from this 

 assumption a tolerable illustration of the subdivision of polarized 

 light by reflexion in an oblique plane," by " supposing the polar 

 motion to be resolved " into two constituents, which fare 

 differently at reflexion. 



In a further letter to Arago, dated April 29th, 1818, Young 

 recurred to the subject of transverse vibrations, comparing light 

 to the undulations of a cord agitated by one of its extremities.^ 

 This letter was shown by Arago to Fresnel, who at once saw 

 that it presented the true explanation of the non-interference 

 of beams polarized in perpendicular planes, and that the latter 

 effect could even be made the basis of a proof of the correctness 

 of Young's hypothesis : for if the vibration of each beam be 

 supposed resolved into three components, one along the ray and 

 the other two at right angles to it, it is obvious from the Arago- 

 Fresnel experiment that the components in the direction of the 

 ray must vanish : in other words, that the vibrations which 

 constitute light are executed in the wave-front. 



It must be remembered that the theory of the propagation 

 of waves in an elastic solid was as yet unknown, and light was 



* Peacock's Life of Young, p. 391. t Young's Works, i., p. 279. 



JThis analogy had been given by Hooke in a communication to the Royal 

 Society on Feb. 15, 1671-2. But there seems no reason to suppose that Hook e- 

 appreciated the point now advanced by Young. 



