1G2 UNDULATORY THEORY OF LIGHT. 



had been suggested, to explain the multiplied and brilliant phenomena of polari- 

 zation and double retraction had been universally felt and acknowledged, that an 

 idea presented itself (and tliis .simultaneously, or nearly so, to two distinguished 

 physicists ) which contained within itself the key to the original and to all suc- 

 ceeding difficulties. In undulation, as understood by Huyghens, Newton, and 

 others, the molecular movements were supposed to be always in the direction of 

 wave-progress and the contrary. Though this is the case with the atmospheric 

 undulations which produce sound, it was ingeniously suggested by Fresnel and 

 Young th it it is probably not so with the ethereal undulations which produce 

 light. Polarized rays are differently affected to reflecting surfaces on their dif- 

 ferent sides. Suppose the movements of the molecules to be normal to the di- 

 rection of progress, and this fact is easily explained. And not only that, but a 

 whole class of perplexing, and pre^viously, in fact, entirely inexplicable phenom- 

 ena, to which we shall have presently to attend, are, by this simple supposition, 

 rendered, not merely intelligible, bat so necessary consequences of the hypothe- 

 sis, that they might have been predicted (as some of them actually were) before 

 having been ever observed. 



Though the idea of transverse vibrations occurred, as just remarked, to Young 

 as well as to Fresnel, it is, nevertheless, to Fresnel alone that the credit is due 

 of having made it the basis of a theory fully wrought out and est :bli.died, as well 

 upon the basis of a thorough mathematical analysis, as of an elaborate and ex- 

 tensive experimental verification. To apply it at present to explain the possi- 

 ble coexistence of two independent waves of different curvature and difierent 

 progressive velocity, originating simultaneously from a common centre in a 

 doubly refracting medium, we have but to make the two assumptions follow- 

 ing, viz : 



1. The molecular movements, being in both of the two waves at right angles 

 to the direction of progress, are performed in planes which are at right angles 

 to each other. 



2. The elasticity of the ether which determines the velocity of a tremor is 

 not the same in all directions within the medium. 



It will presently be more part.cularly shown in what manner those two as- 

 sumptions serve to explain all the phenomena which appeared so unaccountable 

 to Huyghens, as well as many others which were not known to him. 



§ IV. INTERFERENCE. 



We Avill now proceed to apply briefly the theory of undulation to the expla- 

 nation of some of the phenomena which we have heretofore detailed without 

 accounting for. Many, or must of the phenomena depend on the mutual influ- 

 ence of different undulations conspiring or conflicting in consequence of the 

 superposition of one upon the other. A gi'oss illustration often employed in 

 explaining this idea is to refer to the appearances presented by the intersecting 

 rings formed in water into which two pebbles have been thrown. The elevated 

 rings and their intervening depressions are undulations; the molecular move- 

 ments are vertical, while the undulation progress is horizontal. When the rings 

 intersect, the points where two ridges cross are doubly elevated ; the points 

 where two hollows ci'oss are doubly depressed ; while the points in v/hich a 

 ridge in one system crosses a hollow of the other are neither elevated nor de- 

 pressed. The term applied to this influence of one undulation upon another is 

 interj'erence. 



