320 NINETEENTH CENTURY. FT. in. 



another curious effect is seen, namely, a succession of bands 

 appear tinged with the colours of the rainbow ; and this, too, 

 Young showed to be beautifully explained by the Undula- 

 tory Theory. It was stated at p. 177 that the colour of 

 the light which reaches our eye depends upon the rapidity 

 of the vibrations of the ether, just as the sound of a note 

 upon our ear depends upon the rapidity of the vibrations of 

 tne air. Consequently the waves of the prismatic colours 

 are of different lengths, so that when the two rays of light 

 meet behind the card the waves of the various colours do 

 not all arrive together. For example, those waves which 

 cause us to see the colour violet are much shorter and more 

 rapid than those which cause us to see red. Therefore, 

 when the red waves meet each other as friends (as in 

 Fig. 57), and make a strong vibration, the violet ones will 

 meet each other as foes (as in Fig. 58), and interfere with 

 each other ; and so we shall see a bright red stripe made by 

 the strong red wave, while the violet waves will be destroyed. 

 A little farther on the violet waves will meet as friends, and 

 then we see a violet streak, while the red ones will in their 

 turn be destroyed. 



Colours on the Soap-bubble. The beautiful colours 

 of the soap-bubble are caused in this way, and an explana- 

 tion of them will help you to picture to yourself this effect 

 of the interference of light. If you have ever blown a well- 

 shaped soap-bubble, and watched it settle down quietly 

 where there is no wind to disturb it, you cannot fail to have 

 noticed the colours which appear upon it. If the bubble is 

 very perfect, these colours arrange themselves in rings, 

 beginning with a dark spot at the top of the bubble and 

 forming alternate bands of blue, yellow, orange, and red, 

 which grow fainter and fainter down the sides of the bubble 

 till they disappear. The reason of these colours is that, 



