THE LAW OF REFRACTION. 



as you will see at once if you lay a straight ruler from end 

 to end of each ray ; and if we were to draw other rays 

 between these they would all be refracted at different 

 angles, those being most bent which were farthest from the 

 perpendicular. 



Measurement of Refraction in Water. 



w w, Water. A A', B B', Rays passing from air into water, c' c', Line from the ray A 

 to the perpendicular x\ in the water, three-fourths the length of c c from the ray A 

 in the air. d d , d d, Similar lines from the ray B. 



Now in making telescopes it is very important to know 

 how much each ray is refracted ; and as the rays are in- 

 finite in number, it was impossible to know this unless some 

 general rule could be found. Snellius set himself this task, 

 and after a great number of very delicate experiments he 

 arrived at a law which has proved to be always true. This 

 law is best explained by the following experiment, which is 

 not difficult to understand, although it is troublesome to 

 perform it accurately. 



Draw a circle on a black board with an upright line x x 



