134 FKAGMENTS 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. Taking two 

 pieces of glass, the one plane and the other very slightly 

 curved, and pressing both together, he obtained a film 



