io8 SEVENTEENTH CENTURY. rr. in. 



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 per- 

 form it accurately. 



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

 through it, and then place the board upright in a vessel of 

 water so that the surface of the water crosses the centre o. 

 Then pass a ray of light through a tube so placed that the 

 ray falls across the board in the direction A o ; it will then 

 pass on through the water to some point A'. The line o A 

 will now cut the circle at the point c, and the line o A' will 

 cut it at c ' . From these two points draw horizontal lines c c 

 and d d on the board to the upright line x x'. Then if you 

 compare the length of these two lines you will find that c' d 

 in the water is exactly three-fourths of c c in the air. 



Again, if you throw the light from your tube in the direc- 

 tion B <?, the result is the same. The length of d d' in the 

 water will again be three-fourths of d d in the air. And this 

 is equally true of all rays passing from air into water. When 

 a vertical line is drawn through the point where the ray falls 

 on the water, the two horizontal lines drawn to the place where 

 the circle cuts the ray will always be in the same proportion, 

 at whatever angle the ray strikes the water. Therefore, |ths 

 is said to be the index of refraction for water, meaning that 

 every ray which passes from air into water will have these 

 two horizontal lines in the proportion of 4 to 3. In passing 

 from air into glass they would always be in the proportion 

 of 3 to 2, and every different substance, such as ice, amber, 

 diamond, &c., has its own angle of refraction. These have 

 been calculated, and tables made, from which you can learn 



