564 Professor Tyndall [Jan. 22, 



some of the apparatus he employed. I hold in my hand an ancient 

 tract upon this subject by the illustrious Huyghens. It was picked up 

 on a bookstall, and presented to me some years ago by my friend 

 Professor Dewar. In this tract, Huyghens deals with refraction and 

 reflection, giving a complete explanation of both ; and here, also, he 

 enunciated a principle which now bears his name, and which forms 

 one of the foundation stones of the undulatory theory. 



The most formidable obstacle to Young's advance, and one which 

 he never entirely surmounted, was an objection raised by Newton to 

 the assumption of a fluid medium as the vehicle of light. Looking 

 at the waves of water impinging on an isolated rock, Newton observed 

 that the rock did not intercept the wave motion. The waves, on the 

 contrary, bent round the rock, and set in motion the water at the back 

 of it. Basing himself on this and similar observations, he says, " Are 

 not all hypotheses erroneous in which light is supposed to consist of a 

 pression or motion propagated through a fluid medium ? If it consisted 

 in pression or motion, it would bend into the shadow." He instances 

 the case of the sound of a bell being heard behind a hill which conceals 

 the bell ; of the turning of corners by sound ; and then, with con- 

 clusive force he points to the case of a planet coming between a fixed 

 star and the eye, when the star is completely blotted out by the 

 interposition of the opaque body. This, Newton urged, could not 

 possibly occur if light were propagated by waves through a fluid 

 medium, for such waves would infallibly stir the fluid behind the 

 planet, and thus obliterate the shadow. 



Young was firmly persuaded of the truth of the undulatory 

 theory. The number of riddles that, by means of it he had solved, 

 the number of secrets he had unlocked, the number of difficulties he 

 had crushed, rendered him steadfast in his belief; still, he never fairly 

 got over this objection of Newton's ; and it was first set aside by one 

 of the most illustrious men that ever adorned the history of science. 

 A young French officer of engineers, Augustin Fresnel, first really 

 grappled with the difficulty and overthrew it. The principle of 

 Huyghens, to which I have already referred, is, that every particle, 

 in every wave, acts as if it alone were a centre of wave motion. When 

 you throw a stone into the Serpentine, circular waves or ripples are 

 formed which follow each other in succession, retreating further and 

 further from the point of disturbance.* Fix your attention on one of 



* '* I prove it thus, take heed now 

 By experience, for if that thou 

 Threw in water now a stone 

 Well west thou it will make anoue, 

 A little rouudell as a cercle, 

 Peraventure as broad as a couercle, 

 And right anone thou shalt see wele, 

 That whele cercle wil cause another whole. 

 And that a third and so forth brother, 

 Every cercle causing other." 



Chaucer'' s ' House of Fame* 



