CH. XXXII. INTERFERENCE OF LIGHT. 305 



appeared entirely ^ and when he took away his hand they 

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

 in one direction only behind the card it spreads itself out 

 equally, but directly the two sets of rays from the two sides 

 met each other, dark and light bands appeared. 



Now Newton's emission theory would give no explana- 

 tion of this curious fact, for if light were made of tiny par- 

 ticles there is no reason why these particles in crossing each 

 other should make dark bands. On the contrary, the more 

 of them there were the more Hght there ought to be. The 

 Undulatory or Wave Theory, however, explained the bands 

 perfectly, and this we must now try to understand. 



You will remember that Huyghens supposed an ether 

 filling all space to be set in motion by the sun, or any other 

 luminous body, and to heave up and down in tiny waves just 

 as the sea heaves, or the water of a pond when you agitate it. 



Suppose, therefore, that a number of waves of water, all 

 of the same size, are moving along one side of a lake as at a. 

 Fig. 48, p. 306, and flowing out through a narrow channel at 

 the end, and suppose another set of waves to be moving along 

 the other side, b, so that the two sets meet at the mouth of the 

 channel. Then, if the two waves c and d are both rising up 

 when they meet, they will join together into one large wave, 

 and will continue to flow in large waves -down the channel. 

 But if they meet, as in Fig. 49, when c is falling and d is 

 rising, then c will flow into the hollow of d and fill it up, 

 and instead of a large wave being made, the surface of the 

 water will become smooth. 



Now Young pointed out that this is exactly what happens 

 to the undulations of light. After passing through the hole 

 in the shutter, they move on till they come to the card, and 

 here they wheel round each edge of the card and meet 



