THE WAVE THEORY OF LIGHT. 97 



Lucasian professorship, and in his turn he also teaches optics. The 

 pupil soon becomes greater than his teacher, and he gives out this 

 great result: White light which seemed the type of pure light is not 

 homogeneous; it consists of rays of different refrangibility, and he 

 demonstrates it hj the celebrated experiment of the solar spectrum, in 

 which a ray of white light is decomposed into a series of colored rays 

 like a rainbow; each shade of the color is simple, for the prism does not 

 decompose the shade. This is the origin of the spectral analysis. 

 This analysis of white light brought Newton to explain the colors of 

 the thin plates which are, for instance, observed in soap bubbles. 

 The fundamental experiment, that of Newton's rings, is one of the 

 most instructive in optics, while the laws that govern it are of admir- 

 able simplicity. 



The theory was expomided in a discourse addressed to the Royal 

 Society, with the title, "A new hj^pothesis concerning light and color."" 



This discourse called forth from Hooke a sharp complaint. Hooke 

 also had already examined the color of thin plates, and endeavored to 

 explain them in the wave system. He had the merit, which Newton 

 himself readily granted, to substitute for the progressive wave of Des- 

 cartes a vibrating one — a new and extremely important notion. He 

 had even noticed the part of the two reflecting services of the thin 

 plate and the mutual action of the reflected waves. Consequently 

 Hooke should have been the ver^^ forerunner of the modern theory if 

 he had had, as Newton, the clear intelligence of the simple rays. But 

 his vague reasoning to explain the colors takes awa}^ all demonstrative 

 vakie from his theor3\ 



Newton is ver}' afl'ected by this complaint of priority, and combats 

 the arguments of his adversary by remarking that the wave theory is 

 inadmissible because it does not explain the existence of the luminous 

 ra}^ and of the shadows. He denies the opinion that ho has raised a 

 theory ; he certifies that he does not admit either the wave hypothesis 

 or the emission, but he says: 



"He shall sometimes, to avoid circumlocution and to represent it 

 conveniently, speak of it as if he assumed it and propounded it to be 

 believed." 



And, really, in the Proposition XII (second book of his Optics)^ 

 whicli constitutes what was since called the theory of fits, Newton 

 remains absolutely on the ground of facts. He says simply, the phe- 



'Prop. XII. — Every ray of light in its passage through any refracting surface ia 

 put into a certain transient constitution or state, which in the i>rogress of the ray 

 returns at equal intervals, and disposes the ray at every return to be easily trans- 

 mitted through the next refracting surface, and between the returns to be easily 

 reflected by it. (Sir Isaac Newton, Opticks: or a Treatise of the Reflections, Refrac- 

 tions, Inflexions, and Colors of Light. L<)U<lon, 1718. Second edition, with addi- 

 tions, p. 293. ) 



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