520 THE PRINCIPLES OF SCIENCE. [CHAP. 



of these natures to the nature sought, and the uncertain 

 and separable alliance of the other, whereby the question 

 is decided, the former nature admitted for the cause, 

 and the other rejected. These instances, therefore, afford 

 great light, and have a kind of overruling authority, so 

 that the course of interpretation will sometimes terminate 

 in them, or be finished by them." 



The long-continued strife between the Corpuscular and 

 Undulatory theories of light forms the best possible illus- 

 tration of an Experimentum Crucis. It is remarkable in 

 how plausible a manner both these theories agreed with 

 the ordinary laws of geometrical optics, relating to reflec- 

 tion and refraction. According to the first law of motion 

 a moving particle proceeds in a perfectly straight line, 

 when undisturbed by extraneous forces. If the particle 

 being perfectly elastic, strike a perfectly elastic plane, it 

 will bound off in such a path that the angles of incidence 

 and reflection will be equal. Now a ray of light proceeds 

 in a straight line, or appears to do so, until it meets a re- 

 flecting body, when its path is altered in a manner exactly 

 similar to that of the elastic particle. Here is a remark- 

 able correspondence which probably suggested to Newton's 

 mind the hypothesis that light consists of minute elastic 

 particles moving with excessive rapidity in straight lines. 

 The correspondence was found to extend also to the law 

 of simple refraction ; for if particles of light be supposed 

 capable of attracting matter, and being attracted by it at 

 insensibly small distances, then a ray of light, falling on 

 the surface of a transparent medium, will suffer an increase 

 in its velocity perpendicular to the surface, and the law 

 of sines is the consequence. This remarkable explanation 

 of the law of refraction had doubtless a very strong 

 effect in leading Newton to entertain the corpuscular 

 theory, and he appears to have thought that the analogy 

 between the propagation of rays of light and the motion 

 of bodies was perfectly exact, whatever might be the 

 actual nature of light. 1 It is highly remarkable, again, 

 that Newton was able to give by his corpuscular theory, 

 a plausible explanation of the inflection of light as dis- 



1 Principia, bk. i. Sect. xiv. Prop. 96. Scholium. Opticks, Prop. vi. 

 3rd edit. p. 70. 



