CH. xxxin. DR. THOMAS YOUNG. 317 



him mentioned as an Egyptian scholar ; but what we have 

 now to consider are his discoveries about Light. 



Young tells us himself that it was in May 1801 that he 

 first made an experiment which seemed to him to prove 

 that light must be a succession of tiny waves moving across 

 space as Huyghens had supposed. His experiment was the 

 following. He made a hole in the window-shutter of a dark 

 room, and covered it with a piece of thick paper, in which 

 he had pricked a small hole with a needle. He then put a 

 small looking-glass outside the shutter, so as to throw the 

 sunlight very fully upon the hole and send a cone of spread- 

 ing light through it In this cone of light he held a very 

 narrow strip of card and watched the shadow which it threw 

 on the wall, or on another piece of card behind it. On each 

 side of the shadow there were some faint fringes of colour, 

 but besides these he saw in the shadow itself dark and light 

 upright tinted bands, which finished off in a faint white band 

 in the middle of the shadow. It was from these faint bands, 

 which many men would have thought not worth noticing, 

 that Young worked out the truth of the Wave Theory of 

 Light. 



The first question he asked himself was c Why should 

 there be any light at all in the shadow?' This was not 

 difficult to answer, for as light travels in all directions, a part 

 of it, passing on each side of the strip of card, will spread 

 out behind it. But why should this light arrange itself in 

 stripes and not fall equally all over the shadow? It seemed 

 at first impossible to explain this ; but when Young placed 

 his hand so as to prevent the light passing along one of the 

 edges of the card he found that the fringes or bands dis- 

 appeared entirely, and when he took away his hand they re- 

 turned. It was clear, then, that so long as the light passed 

 in one direction only behind the card it spread itself out 



