40 THE FOUNDATIONS OF SCIENCE 



into its coloured constituents by means of a prism. 

 Newton felt the necessity of imagining space to be 

 filled with a subtle medium or aether, though the 

 difficulty of accounting by any then known theory 

 of waves for the transfer of light in straight lines or 

 rays led him to suppose in addition that light con- 

 sisted of streams of minute particles or corpuscles shot 

 off from the luminous body with immense velocity. 



The pure wave theory, however, had already been 

 developed by Huygens (1620-1695), but it was not 

 till a century later that Young (1773-1829) and 

 Fresnel (1788-1827) succeeded in establishing it in 

 general estimation. In their hands the new theory 

 explained satisfactorily all the phenomena then 

 known, especially those of the interference of two 

 rays of light to produce coloured fringes. When two 

 waves come together, if their crests coincide their 

 joint effect will be the sum of their individual effects ; 

 but, if the crests of one wave coincide with the 

 troughs of the other, their effects will be opposite and 

 may destroy each other. Hence some constituents 

 of a dual beam of white light may be destroyed, and 

 colour appear as the result of the remainder 



Similarly, when the wave-length is very small 

 compared with the dimensions of the obstacles or 

 the distances concerned, the fact that light travels 

 in straight lines is explained by the interference of 

 all parts of the possible wave-front except a small 

 area which forms an advancing ray. 



