1886.] on Thomas Young. 573 



of natural problems demanding solution, and after a temporary lull 

 the desire to know more of the nature of light grew in force. New 

 stars arose in France, while the strenuous industry and experimental 

 discoveries of Brewster did much to hold us in equipoise with the 

 Continent. In Paris, La Place, Malus, Biot, and Arago were all 

 actively engaged. The three first proceeded strictly on the New- 

 tonian lines, and by the memoir of La Place, on Double Kefraction, 

 all antagonism to the theory of emission was considered to be for ever 

 overthrown. In the ' Quarterly Eeview,' Young criticised this memoir 

 with sagacity and power, and his criticism remains valid to the present 

 time. In accordance with the principles of the wave theory, Huyghens 

 had given a solution of the problem of double refraction in Iceland spar. 

 The solution was oj)posed to that of La Place. Dr. Wollaston, a man 

 of the highest scientific culture and the most delicate experimental 

 skill, subjected the theory of Huyghens to the severest metrical tests, 

 and his results proved entirely favourable to that theory. Wollaston, 

 however, lacked the boldness which would have made him a com- 

 mander in those days of scientific strife. He saw opposed to him the 

 names of Newton and La Place, and in the face of such authority he 

 shrank from closing with the conclusions to which his own experi- 

 ments so distinctly pointed. 



We now come to a critical point in the fortunes of the wave 

 theory. I need not again refer to the difference between the motion 

 of a wave and the motions of the particles which constitute a wave. 

 A wave of sound, for instance, passing through the air of this room 

 would have a velocity of about 1100 feet a second, while the particles 

 which constitute the wave, and propagate it at any moment, may only 

 move through inconceivably small spaces to and fro. Now, in the case 

 of sound, this to-and-fro motion occurs in the direction in which the 

 sound is propagated, and a little reflection will make it clear that no 

 matter how a ray of sound, if we may use the term, is received upon 

 a reflecting surface, it would be echoed equally all round as long as 

 the angle inclosed between the reflecting surface and the ray remains 

 unchanged. In other words, the sound ray has no sides and no 

 preferences, as regards reflection. Now, Malus discovered that in 

 certain conditions a beam of light shows such preferences. When 

 caused to impinge upon a plane glass mirror, placed in a certain 

 position, it may be wholly reflected; whereas when the mirror is 

 placed in the rectangular position it may not be reflected at all. 



Up to the hour when this discovery was made by Malus, light 

 had been supposed to be propagated through ether exactly as sound 

 is propagated through air. In other words, the direction in which the 

 particles of ether were supposed to vibrate to and fro coincided with 

 that of the ray of light. Those who had previously held the undu- 

 latory theory were utterly staggered by this new revelation, and their 

 perplexity was shared by Young. He was for a time unable to conceive 

 of a medium capable of propagating the impulses of light in a way 

 different from the propagation of the impulses of sound. To ascribe 



