150 THE PRINCIPLES OF SCIENCE. 



between two equally plausible views. ' When in search 

 of any nature, the understanding comes to an equilibrium, 

 as it were, or stands suspended as to which of two or 

 more natures the cause of nature inquired after should 

 be attributed or assigned, by reason of the frequent and 

 common occurrence of several natures, then these Crucial 

 Instances show the true and inviolable association of one 

 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 the need of an Experimentum Crucis. It is 

 highly remarkable in how complete and plausible a 

 manner both these theories agreed with the ordinary laws 

 of geometrical optics, relating to reflection and refraction. 



A moving particle, according to the first law of motion, 

 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 perfectly straight line, 

 or appears to do so, until it meets a reflecting body, when 

 its path is altered in a manner exactly similar to that of 

 the elastic particle. Here is a remarkable correspondence 

 which probably suggested to Newton's mind that light 

 consisted 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 these 

 particles of light be supposed capable of attracting matter, 

 and being attracted by it at insensibly small distances, 



