100 THE WAVE THEORY OF LIGHT. 



as it was, hj the great mathematician, who had the glory of submit- 

 ting the motions of all celestial bodies to the one law of universal 

 gravitation. 



To-day this theory is abandoned; it is condemned by the experi- 

 iiientmn cruets of Arago, realized by Fizeau and Foucault. One ought, 

 however, to acknowledge that it has constituted a real progress ])y the 

 precise and new notions which it contains. The ra^' of light, considered 

 up till then, was simply the trajectory of a particle in rectilinear 

 motion; the ray of light, such as Newton described it, possesses a reg- 

 ular periodic structure, and the period or interval of fits, characterizes 

 the color of the ray. This is an important result. It only requires a 

 more suitable interpretation to transform the luminous ray into a 

 vibratory wave; but we had to wait a centur3% and Dr. Thomas Young, 

 in 1801, had the honor of discovering it. 



Resuming the study of thin plates, Thomas Young shows that every- 

 thing is explained with extreme simplicity, if it be supposed that the 

 homogeneous linninous ray is analogous to the sonorous wave produced 

 by a nuisical sound; that the vibrations of ether ought to compose — 

 that is to say, to interfere — according to the expression that he pro- 

 poses as to their mutual actions. 



Although Young liad taken the clever precaution of supporting his 

 views l)y the authority of Newton,^ the hypothesis found no favor; his 

 principle of interference led to this singular result, that light added to 

 light could, in certain cases, produce darkness, a paradoxical result 

 contradicted l)v daily experience. The only verification that Young 

 brought forward was the existence of dark rings in Newton's experi- 

 ment; darkness due, according to him, to the interference of waves 

 reflected on the two faces of the plate. But as the Newtonian theory 

 interpreted the fact in a different manner, the proof remained doubt- 

 ful, an ci-ptvimentmii cruch was wanting. Young did not have the 

 good success to obtain it. 



The theor}' of waves relapsed then once more into the obscurity of 

 controversy, and the terrible argument of the rectilinear propagation 

 was raised afresh against it. The most skilled geometers of the 

 period — Jjaplace, Biot, Poisson — naturally leaned to the Newtonian 

 opinion; Laplace in particular, the celebrated author of the Mecanique 

 Celeste, had even taken the offensive. He was going to attack the 

 theory of waves in its most strongly fortified intrenchments, which 

 had been raised b}^ the illustrious Huygens. 



Huygens, indeed, in his "Traite de la Lumiere," had resolved a prob- 

 lem before which the theory of emission had remained mute; that is 

 to say, the explanation of the double refraction of Iceland spar. The 

 wave theory (on the contrary) reduced to the simplest geometrical 



^The Bakerian lecture "On the theory of hght and colors," by Thomas Young. 

 Phil. Trani?. of the Royal Society for the year 1802, 



^ 



