THE SKf 143 



already discoursed of "a very quick motion that causes 

 light, as well as a more robust that causes heat." New- 

 ton had ascribed the sensation of red to the shock of his 

 grossest, and that of violet to the shock of his finest, lumi- 

 niferous projectiles. Denning the one, and displacing the 

 other of these notions, the wave-theory affirms red to be 

 produced by the largest, and violet by the smallest waves 

 of the visible spectrum. The theory of undulation had to 

 encounter that fierce struggle for existence which all great 

 changes of doctrine, scientific or otherwise, have had to 

 endure. Mighty intellects, following the mightiest of them 

 all, were arrayed against it. But the more it was discussed 

 the more it grew in strength and favor, until it finally sup- 

 planted its formidable rival. No competent scientific man 

 at the present day accepts the theory of emission, or refuses 

 to accept the theory of undulation. 



Boyle and Hooke had been fruitful experimenters on 

 those beautiful iridescences known as the "colors of thin 

 plates.'* The rich hues of the thin-blown soap-bubble, of 

 oil floating on water, and of the thin layer of oxide on 

 molten lead, are familiar illustrations of these iris colors. 

 Hooke showed that all transparent films, if only thin 

 enough, displayed such colors; and he proved that the 

 particular color displayed depended upon* the thickness of 

 the film. Passing from solid and liquid films to films 

 of air, he says: "Take two small pieces of ground and 

 polished looking-glass plate, each about the bigness of a 

 shilling; take these two dry, and with your forefingers 

 and thumbs press them very hard and close together, and 

 you shall find that, when they approach each other very 

 near, there will appear several irises or colored lines." 

 Newton, bent on knowing the exact relation between the 



