xxyi.] CHARA.CTER OF THE EXPERIMENTALIST. 5S5 



number of foci, which prevent a clear view from being ob- 

 tained at any point. 



What astonishes the reader of the OpticJcs is the per- 

 sistence with which Newton follows out the consequences 

 of a preconceived theory, and tests tlie one notion by a 

 ^\•onderful variety of simple comparisons with fact. The 

 ease with which he invents new combinations, and fore- 

 sees the results, subsequently verified, produces an insuper- 

 able conviction in the reader that he has possession of the 

 truth. And it is certainly the theory which leads him to 

 the experiments, most of which could hardly be devised by 

 accident. Newton actually remarks that it was by mathe- 

 niatically determining all kinds of phenomena of colours 

 which could be produced by refraction that he liad " in- 

 vented " almost all the experiments in the book, and he 

 promises tiiat others who shall " argue truly," and try the 

 experiments with care, will not be disappointed in the 

 results.^ 



The philosophic method of Huyghens was the same as 

 that of Newton, and Huyghens' investigation of double 

 refraction furnishes almost equally beautifid instances of 

 theory guiding experiment. So far as we know double re- 

 fraction was first discovered by accident, and was described 

 by Erasmus Bartholinus in 1669. The phenomenon then 

 appeared to be entirely exceptional, and the laws governing 

 the two paths of the refracted rays were so unapparent 

 and complicated, that Newton altogether misunderstood the 

 phenomenon, and it was only at the latter end of the last 

 century that scientific men 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 

 Trait6 de la LuinUre he pointed out that, in ordinary 

 refraction, the velocity of propagation of tlie wave is 

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

 wave is spherical, and reaches equal distances in equat 

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

 be of unequal elasticity in diflereut directions, so that a 

 disturbance would reach unequal distances in equal times, 

 and tlie wave produced would have a spheroidal form. 



' U^tickc, bk. i. part ii. I'rop. 3. 3rd cd. p. 115. 



