CH. XXL DOUBLE REFRACTION OF LIGHT. 179 



will reach the velvet 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 before, and thus the runner will swing round or be refracted 

 into the direction b c. Then, as it passes out again, the 

 left wheel will come out of the velvet first and will move 

 more quickly on the smooth board, while the right is still 

 checked by the velvet ; therefore the runner will again be 

 shifted round or refracted as it passes out into the direction c d. 

 In figure 2, on the other hand, even if the runner passes in 

 vertically, yet the rounded surface of the lens causes the left 

 wheel to enter first, and as, in consequence of the rounding 

 of the second surface this same wheel comes out last, the ray 

 is refracted inwards along its whole course. You can easily 

 follow the course of the runner through the other lens 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 from one medium to another. 

 Double Refraction. There is another remarkable fact 

 about light which Huyghens explained ; namely, the double 

 refraction of light through a crystal called Iceland spar. A 

 physician of Copenhagen named Erasmus Bartholinus had 

 received from Iceland a crystal in the form of a rhomboid 

 (see Fig. 36), which, when broken, fell 

 into pieces of the same shape. Bartho- 

 linus called this crystal ' Iceland spar,' 

 and while making experiments with it FIG. 3 6. 



he observed that an inkspot or any small A spot of ink seen through 

 object seen through it appeared to be a crystal of Iceland spar " 

 doubled. He was not able to explain this curious fact, but he 

 published an account of it in 1669, and Huyghens accounted 

 for it quite correctly by suggesting that the crystal was more 

 14 



