280 THOMAS YOUNO. 



country. After Young's crushing reply^ which produced 

 no effect whatever upon the public, the author of that 

 reply was practically forgottten as a factor in the ad- 

 vancement of physical optics. But science has always 

 before her the stimulus 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 ex- 

 perimental discoveries of Brewster did much to hold us 

 in equipoise with the Continent. In Paris, Laplace, 

 Malus, Biot, and Arago were all actively engaged. The 

 first three proceeded strictly on Newtonian lines, and 

 by the memoir of Laplace on Double Refraction all 

 antagonism to the theory of emission was considered to 

 be for ever overthrown. In the ' Quarterly Review,' 

 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 opposed 

 to that of Laplace. Dr. Wollaston, a man of the 

 highest scientific culture and the most delicate experi- 

 mental 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 commander 

 in those days of scientific strife. He saw opposed to 

 him the names of Newton and Laplace, and in the 

 face of such authority he shrank from closing with the 

 conclusions to which his own experiments so distinctly 

 pointed. 



We now come to a critical point in the fortunes of 

 the wave theory. I need not again refer to the differ- 

 ence between the motion of a wave and the motions of 

 the particles which constitute a wave. A wave of 



