134 FRAGMENTS OF SCIENCE. 



ascribed the sensation of red to the shock of his grossest, 

 and that of violet to the shock of his finest luminiferous 

 projectiles. Defining the one, and displacing the other 

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

 duced 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 

 favour, until it finally supplanted 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 ' colours 

 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 colours. Hooke showed that all transparent films, 

 if only thin enough, displayed such colours; and he 

 proved that the particular colour 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 coloured lines.' Newton, bent on knowing the 

 exact relation between the thickness of the film and the 

 colour it produced, varied Hooke's experiment. Tak- 

 ing two pieces of glass, the one plane and the other 

 very slightly curved, and pressing both together, he ob- 



