304 NINETEENTH CENTURY. pt. ill. 



very famous as one of the first men who deciphered the 

 Egyptian hieroglyphical writings, and you will often hear 

 him mentioned as an E^ptian 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 

 spreading 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 //Ifd'^ dark and 

 light upright 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 — * 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- 



