CII. XIII. 



THE LAW OF REFRACTION, 



107 



angles at which rays are refracted in water and glass, but 

 they did not know of any law by which they could calculate 

 how much any particular ray would be bent out of its course. 

 For instance, in Fig. 1 2, suppose w w to be the surface of 

 water in a glass vessel, upon which the rays A and B fall at 



13 



Measurement of Refraction in Water. 



W w, Water. A A', B B', Rays passing from air into water, d </, Line from the ray A 

 to the perpendicular jc 1 , in the water, three-fourths the length of c c from the ray A 

 in the air. d! d 1 , d d, Similar lines from the ray B. 



the point o, and are refracted A to A' and B to B'. It is evi- 

 dent that B is bent much more out of its course than A, 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. 

 Now in making telescopes it is very important to know 

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

 in number, it was impossible to know this unless some 



