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 fa,vor, 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 appear 
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 
