470 THE PRINCIPLES OF SCIENCE. [CHAP. 



can never be followed into all their ultimate results. 

 Young explained the production of Newton's rings by 

 supposing that the rays reflected from the upper and 

 lower surfaces of a thin film of a certain thickness were in 

 opposite phases, and thus neutralised each other. It was 

 pointed out, however, that as the light reflected from the 

 nearer surface must be undoubtedly a little brighter than 

 that from the further surface, the two rays ought not to 

 neutralise each other so completely as they are observed 

 to do. It was finally shown by Poisson that the dis- 

 crepancy arose only from incomplete solution of the 

 problem ; for the light which has once got into the film 

 must be to a certain extent reflected backwards and 

 forwards ad infinitum ; and if we follow out this course of 

 the light by perfect mathematical analysis, absolute dark- 

 ness may be shown to result from the interference of 

 the rays. 1 In this case the natural laws concerned, those 

 of reflection and refraction, are accurately known, and 

 the only difficulty consists in developing their full 

 consequences. 



Discovery of Hypothetically Simple Laws. 



In some branches of science we meet with natural laws 

 of a simple character which are in a certain point of view 

 exactly true and yet can never be manifested as exactly 

 true in natural phenomena. Such, for instance, are the 

 laws concerning what is called a perfect gas. The gaseous 

 state of matter is that in which the properties of matter 

 are exhibited in the simplest manner. There is much 

 advantage accordingly in approaching the question of 

 molecular mechanics from this side. But when we ask 

 the question What is a gas ? the answer must be a 

 hypothetical one. Finding that gases nearly obey the 

 law of Boyle and Mariotte ; that they nearly expand by 

 heat at the uniform rate of one part in 272-9 of their 

 volume at o for each degree centigrade ; and that they 

 more nearly fulfil these conditions the more distant the 

 point of temperature at which we examine them from 

 the liquefying point, we pass by the principle of con- 



1 Lloyd's Lectures on the Wave Theory, pp. 82, 83. 



