CHARACTER OF THE EXPERIMENTALIST. 231 



It is certainly the theory which leads him to the experi- 

 ments, and most of these could hardly be devised by 

 accident. The fertility with which he invents new combi- 

 nations, and foresees the results, subsequently verified, 

 produces an invincible conviction in the reader that he 

 has possession of the truth. Newton actually remarks 

 that it was by mathematically determining all kinds of 

 phenomena of colours which could be produced by refrac- 

 tion that he had ' invented ' almost all the experiments in 

 the book, and he promises that others who shall ' argue 

 truly,' and try the experiments with care, will not be 

 disappointed in the results. 



The philosophic method of Huyghens was almost ex- 

 actly the same as that of Newton, and Huyghens' investi- 

 gation of the laws of double refraction furnishes almost 

 equally beautiful instances of theory guiding experiment. 

 Double refraction was first discovered by accident, so far 

 as we know, and was described by Erasmus Bartholinus 

 in 1 669. The phenomenon then appeared to be entirely ex- 

 ceptional, and the laws governing the two separate paths 

 of the refracted rays were so unapparent and complicated, 

 that even New ton altogether misunderstood the pheno- 

 menon, and it was only at the latter end of the last century 

 that scientific men generally began to comprehend its laws. 



Nevertheless, Huyghens had, with rare genius, arrived 

 at the true theory as early as 1678. He regarded light 

 as an undulatory motion of some medium, and in his 

 * Traite de la Lumiere,' he pointed out that, in ordinary- 

 refraction, the velocity of propagation of the wave is 

 equal in all directions, so that the front of an advancing 

 wave is spherical, and reaches equal distances in equal 

 times. But in crystals, as he supposed, the medium would 

 be of unequal elasticity in different directions, so that a dis- 

 turbance would reach unequal distances in equal times, and 

 the wave produced would have a spheroidal form. Huy- 



