DISCOVERY BY MEANS OF INFERENCE. 589 



tbat the motions of the satellites were circular and uni- 

 form. Their irregularities disguised the fact in question. 

 As the irregularities became clearly known, Koemer's dis- 

 covery was finally established, and the " Equation of Light" 

 took its place in the Tables.' l 



6 We must consider Descartes in particular as the 

 genuine author of the explanation of the rainbow.' ' There 

 are two main points of this theory, namely, the showing 

 that a bright circular band, of a certain definite diameter, 

 arises from the great intensity of light returned at a 

 certain angle ; and the referring the different colours to the 

 different quantity of the refraction ; and both these steps 

 appear indubitably to be the discoveries of Descartes.' 

 And he informs us that these discoveries were not made 

 without some exertion of thought. ' At first,' he says, ' I 

 doubted whether the iridal colours were produced in the 

 same way as those in the prism ; but, at last, taking my 

 pen, and carefully calculating the course of the rays which 

 fall on each part of the drop, I found that many more 

 come at an angle of forty-one degrees than either at a 

 greater or a less angle ; so that there is a bright bow ter- 

 minated by a shade, and hence the colours are the same 

 as those produced through a prism.' 2 



Newton, about the year 1671, appears to have been 

 the first to infer that the thickness of thin laminae might 

 be discovered by means of their colours. Huyghens, also, 

 soon afterwards concluded that light, like sound, is not 

 a substance, but a vibration, and must therefore require 

 an elastic medium to transmit it ; it was thus that he was 

 led to infer the existence of the universal ether which 

 pervades all bodies and all space. Encke, in his exa- 

 mination of his comet, found a diminution of the periodic 



1 Whewell, History of the Inductive Sciences, 3rd edit. vol. ii. p. 199. 



2 Ibid. p. 280. 



