3 i8 NINETEENTH CENTURY. PT. in. 



y i I ___________ . 



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 could 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 light 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- 57> P- 3 T 5> an d 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. 58, 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 soon 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 trie card, and 

 here they wheel round each edge of the card and meet 

 behind it. Those which meet in the middle of the shadow 

 have each travelled exactly the same distance with the same 

 number of waves, so they meet as in Fig. 57, and a strong 

 undulation is produced, causing a band of light But on 



