^ ISAAC NEWTON 105 



account of the phenomena, he concludes that along the path 

 of a light ray there must be states, or 'fits' as he calls them, 

 of periodic change of different kinds, which he regards as 

 'fits of easy reflection and easy transmission,' and he is able 

 to measure the very small distances apart of these states, 

 and finds it greatest for red light and smallest for violet 

 light. This is the fundamental fact which he deduces from 

 observation in a great variety of experiments. He remarks: 

 'What kind of action or disposition this is; whether it 

 consists in a circulating or a vibrating motion of the Ray, 

 or of the Medium, or something else, I do not here 

 enquire.'^ 



Further advance was actually only assured by quite new 

 experiments made a hundred and thirty years later by 

 Fresnel. Newton carefully avoided dealing with questions 

 which were not directly connected with a comprehensive 

 and clear description of the phenomena he had observed. 

 In particular, he did not propound any theory of light. 

 Quite in opposition to what Newton himself plainly says in 

 the second edition of the Opticks in 17 17, we quite common- 

 ly meet even to-day, and the assertion that he did pro- 

 pound a theory. He did not even set up any definite 

 hypotheses concerning the nature of light. He says at the 

 beginning of the first book of his Opticks: 'My Design in 

 this Book is not to explain the Properties of Light by 

 Hypotheses, but to propose and prove them by Reason and 

 Experiments.' In the second book, in which the colours of 

 thin plates are especially dealt with, he describes a series of 

 twenty-four experiments, and follows these by notes and 

 analogies, and then gives thirteen further related observa- 

 tions. In the third book, which he expressly states in the 

 preface to be unfinished, he brings still further observations, 

 which later became of importance, on the diffraction of light, 

 and closes with thirty-one questions concerning a large 



1 Opticks, book 2, part 3, prop. 12. 



