io6 SEVENTEENTH CENTURY. PT. ill. 



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 /. from these two points draw horizontal lines c c 

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

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

 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 o, 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 pro- 

 portion^ at whatever angle the ray strikes the water. There- 

 fore, fths 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, etc., has its own index of refraction. 

 These have been calculated, and tables made, from which 

 you can learn at once what is the index of refraction for 

 any particular substance. 



It was this law of the proportion between the two hori- 

 zontal lines in the air and in the denser substance which 

 Snellius discovered, which is called after him ' Snell's law.' 

 It is expressed in mathematical language, thus : * The ratio 

 between the sines of the incident and refracted rays is always 

 tiu same for the same substance;' sine being a mathematical 



