244 FRESNEL ON DOUBLE UEFRACTION, 



against that of emission, compel us to recognise this character 

 in the luminous vibrations, it is safer to trust ourselves here to 

 experiment than to the notions, unfortunately too incomplete, 

 hitherto presented to us by the calculations of geometers on the 

 vibrations of elastic, fluids. Before showing how we may con- 

 ceive the propagation of these transversal vibrations in an elastic 

 fluid such as that by which light is transmitted, I must prove 

 that their existence becomes a necessary consequence of facts as 

 soon as the system of waves is admitted. 



When M. Arago and myself had remarked that rays polar- 

 ized at right angles always produce the same quantity of light 

 by their reunion, whatever be their difference of route, I thought 

 that this particular law of the interference of polarized raj^s 

 might be easily explained by supposing that the luminous vibra- 

 tions, instead of pushing the aetherial molecules paralbl to the 

 rays, caused them to oscillate in perpendicular directions, and 

 that these directions were at right angles to each other for two 

 beams polarized at a right angle. But this supposition was so 

 contrary to the received ideas on the nature of the vibrations of 

 elastic fluids, that I was a long time before adopting it entirely; 

 and even when the assemblage of facts and new reflections had 

 convinced me that it was necessary to the explanation of the 

 phaenomena of optics, I waited till I had assured myself that it 

 was not contrary to the principles of mechanics before sub- 

 mitting it to the examination of philosophers. Mr. Young, more 

 bold in his conjectures and less confiding in the views of geo- 

 meters, has published it before me (although perhaps he thought 

 of it after me) ; and therefore the priority belongs to him with 

 regard to this theoretical view, as on many others. It was the 

 experiments of Sir David Brewster on bi-axal crystals which led 

 him to think that the vibrations of light, instead of being executed 

 longitudinally, in the direction of the rays, might in truth be 

 transversal, and similar to the undulations of an indefinite cord 

 agitated by one of its extremities. It was, at any rate, on the 

 occasion of Sir David Brewster's observations that he published 

 this hypothesis, that is to say three years after the discovery of 

 the particular characteristics of the interference of polarized rays. 

 Resting on the first law of interference of these rays, I shall en- 

 deavour to prove that the luminous vibrations are performed 

 solely in a direction parallel to the surface of the waves. 



