CH. XXI, DOUBLE REFRACTION OF LIGHT. 



79 



the ether. Let your board be about 2\ feet long, and at one 

 end of it glue on pieces of thick-piled velvet of the shape 

 of lenses (see i, 2, 3, Fig. 2)'^)- ^^t your runner first 



Fig. y^. 



go Straight down the board 

 upon the velvet; it will then 

 run through the velvet with- 

 out changing its course, as a 

 vertical ray does through a 

 lens. Then start it obliquely 

 across the board so that it will 

 reach the lens i in the position 

 B, Here the left wheel of the 

 runner will touch the velvet 

 first, and will be checked by 

 the rough pile, while the right 

 wheel moves on quickly as be- 

 fore, and thus the runner will 

 swing round or be refracted 

 towards the thick part of the 

 lens. Then, as it passes out 

 again the right wheel will come 

 out of the velvet first and will 

 move more quickly on the 

 smooth board, while the left is still checked by the velvet at 

 c ; therefore the runner will again be shifted round or re- 

 fracted as it passes out You can easily follow the course 

 of the runner through the other lenses for yourself, always 

 noticing that the arrow marks which way the ray of light is 

 coming; and when you have done this you will have a 

 beautiful imitation of the way in which the waves of light 

 are refracted in passing through diiferent mediums. 



Double Refraction, — There is still one more remarkable 



.::%'" 



Figures illustrating the passage of the 

 waves of light through clifrer«nt- 

 shaped lenses (TylorX 



