414 HISTORY OF OPTICS. 



glass; and in other cases. He perceived also 1 , that 

 the production of each colour required a plate of 

 determinate thickness, and he employed this cir- 

 cumstance as one of the grounds of his theory of 

 light. 



Newton took up the subject where Hooke had 

 left it; and followed it out with his accustomed 

 skill and clearness, in his Discourse on Light and 

 Colours, communicated to the Royal Society in 

 1675. He determined, what Hooke had not ascer- 

 tained, the thickness of the film which was re- 

 quisite for the production of each colour ; and in 

 this way explained, in a complete and admirable 

 manner, the coloured rings which occur when two 

 lenses are pressed together, and the scale of colour 

 which the rings follow ; a step of the more con- 

 sequence, as the same scale occurs in many other 

 optical phenomena. 



It is not our business here to state the hypo- 

 thesis with regard to the properties of light which 

 Newton founded on these facts ; the " fits of easy 

 transmission and reflection." We shall see here.- 

 after that his attempted induction was imperfect; 

 and his endeavour to account, by means of the laws 

 of thin plates, for the colours of natural bodies, 

 is altogether unsatisfactory. But notwithstanding 

 these failures in the speculations on this subject, 

 he did make in it some very important steps ; for 

 he clearly ascertained that when the thickness of 



1 Micrographia, p. 53. 



