in the Seventeenth Century. 21 



Newton devoted considerable attention to the colours of 

 thin, plates, and determined the empirical laws of the 

 phenomena with great accuracy. In order to explain them, he 

 supposed that " every ray of light, in its passage through any 

 refracting surface, is put into a certain transient constitution or 

 state, which, in the progress of the ray, returns at equal 

 intervals, and disposes the ray, at every return, to be easily 

 transmitted through the next refracting surface, and, between 

 the returns, to be easily reflected by it."* The interval 

 between two consecutive dispositions to easy transmission, or 

 " length of fit," he supposed to depend on the colour, being 

 greatest for red light and least for violet. If then a ray of 

 homogeneous light falls on a thin plate, its fortunes as regards 

 transmission and reflexion at the two surfaces will depend on 

 the relation which the length of fit bears to the thickness of 

 the plate ; and on this basis he built up a theory of the colours 

 of thin plates. It is evident that Newton's "length of fit" 

 corresponds in some measure to the quantity which in the 

 undulatory theory is called the wave-length of the light ; but 

 the suppositions of easy transmission and reflexion were soon 

 found inadequate to explain all Newton's experimental results 

 .at least without making other and more complicated additional 

 assumptions. 



At the time of the publication of Hooke's Micrographia, and 

 Newton's theory of colours, it was not known whether light 

 is propagated instantaneously or not. An attempt to settle 

 the question experimentally had been made many years 

 previously by Galileo,f who had stationed two men with 

 lanterns at a considerable distance from each other ; one of 

 them was directed to observe when the other uncovered his 

 light, and exhibit his own the moment he perceived it. But 

 the interval of time required by the light for its journey was 

 too small to be perceived in this way ; and the discovery was 



* Optic ks, Book ii., Prop. 12. 



t Discorri e dimostrazioiti matemaliche, p. 43 of the Elzevir edition of 1638. 



