THE SKY. 101 



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 

 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 "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, dis- 

 played such colors; and he proved that the particular color 

 displayed depended upon the thickness of the film. Pass- 

 ing 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 appeal- 

 several irises or colored lines." Newton, bent on knowing 

 the exact relation between the thickness of the film and the 

 color 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 of 

 air of gradually increasing thickness from the place of con- 

 tact outward. As he expected, he found the place of con- 

 tact surrounded by a series of colored circles, still known 

 all over the world as " Newton's rings." The colors of his 

 first circle, which immediately surrounded a black central 

 spot, Newton called " colors of the first order; " the colors 

 of the second circle, " colors of the second order," and so 

 on. With unrivaled penetration and apparent success, he 

 applied his theory of "fits" to the explanation of the 

 " rings." Here, however, the only immortal parts of his 

 labors are his facts and measurements; his theory has dis- 

 appeared. It was reserved for the illustrious Thomas 

 Young, a man of intellectual caliber resembling that of 



