THE RENAISSANCE 131 



way, and the rate of variation of x with y written as 

 dx/dy or x. These conceptions may be said to be 

 the starting-point of the whole vast structure of 

 modern pure and applied mathematics. 



Newton also took the lead in the development of 

 physical optics. In 1669, when he was appointed 

 to a professorship at Cambridge, he chose optics as 

 the first subject of his lectures and researches. The 

 discovery of the decomposition of white light by the 

 prism, and the explanation of the rainbow, soon 

 followed, as did the invention of a new form of re- 

 flecting telescope. Two theories of light were known. 

 On the corpuscular or emission theory, luminous 

 bodies emit a stream of minute particles or " cor- 

 puscles " which by impact affect the eye. On the 

 other theory, light consists of waves or undulations, 

 and on this view it appeared necessary to revive a 

 conception which we owe originally to the Greek philo- 

 sophers ; and thus it has been usual, till lately, to 

 invent a subtle medium or aether, which is imagined to 

 possess certain material properties and to fill all space. 

 Owing to the difficulty of explaining the straight path 

 of light, Newton inclined to the corpuscular theory, 

 while Huygens did much to develop the theory of 

 waves. It is certain that the great weight of Newton's 

 name helped to prevent the earlier acceptance of 

 Huygens' views another instance of the danger of too 

 much reliance on authority. But the recent undoubted 

 discovery of the existence of moving particles, closely 

 resembling Newtonian corpuscles, and shot off from 

 radio-active bodies with velocities approaching that 

 of light, has once more made clear the almost super- 

 human insight into nature possessed by Newton's mind. 



