OPTICS. 



water, so as to give it a slight mistiness, 

 the ray B D will be distinctly perceived, 

 and it will be seen that it is a straight 

 line, and that the bending or change in 

 its direction has been produced wholly 

 at the point B in the surface of the water. 

 This bending of the ray A N B is called 

 refraction, from a Latin word, which 

 signifies breaking back, because the 

 ray A B seems to be broken back 

 from its course at B, and the water 

 is said to refract, or break back the ray 

 AB. 



If, in place of fresh water, w r e pour in 

 salt water, it will be found that the ray 

 A B is more bent at B. In like manner 

 alcohol will refract the ray A B more than 

 salt water ; and oil more than alcohol. 

 If we were to cut a piece of glass of the 

 exact shape of the water srOP, and 

 place it in the same way in the vessel, 

 we should find that it would refract the 

 light still more than oil, and in the line 

 Brf. 



Hence we may conclude in general, 

 that when a ray of light passing through 

 air falls in a slanting direction upor the 

 surface of liquid, or of solid bodies, 

 through which light can pass, it is re- 

 fracted by them, and by different bodies 

 in different degrees. 



If, when the vessel M N O P is empty, 

 we fix at C a bright object, such as a 

 sixpence, and place the eye at A in the 

 straight line A N C, the sixpence 

 will be distinctly seen, because one 

 of the rays C N, which proceed from 

 it, must enter the eye at A. Let 

 water be now poured into the vessel 

 till it stand at s r, then the eye at A 

 will no longer see the sixpence ; but if 

 we move the sixpence from C to D, it 

 will become visible to the eye at A the 

 instant it comes to D. Now as the ray 

 from the sixpence at D must pass to the 

 eye in a straight line after it comes out 

 of the water, it must pass in the direc- 

 tion B N A ; and consequently, the ray 

 from the sixpence D, by which it was 

 seen at A, must have been D B, and 

 this ray, in coming out of the water, must 

 have been bent or refracted at B into 

 the line B A. The same effect will be 

 produced if s r is the surface of salt 

 water, alcohol, oil, or glass ; but with 

 these substances we must push the six- 

 pence beyond D towards O, in order 

 that it may be seen at A. 



Hence we may conclude, that when a 

 ray of light, passing through a liquid or 

 a solid body in a direction slanting or 

 oblique to its surface, quits it, it is re- 



fracted by that body, and by different 

 bodies in different degrees. 



Having thus discovered, by very sim- 

 ple experiments, the nature of the re- 

 fraction of light when passing out of a 

 rare or thin medium such as the air into 

 a dense medium such as w r ater, and also 

 out of a dense medium into a rare 

 medium, we must now endeavour to 

 determine the law or rule which the 

 refraction follows when it enters or 

 quits the water at different degrees of 

 obliquity. 



For this purpose, let us describe (Jig. 

 2.) a circle P R Q S upon a piece of 



slale or metal, and having drawn two 

 diameters P C Q, R C S perpendicular 

 to each other, let a small tube A C be 

 attached to the plate, so that it can move 

 freely round C. The plate PRQS 

 must now be placed in a vessel of water, 

 and fixed so that the surface of the 

 water coincides with the line R S, and 

 does not touch the lower end C of the 

 tube A C. Let us now bring the tube 

 A C into the position P C, and t make a 

 ray of light pass down through the tube, 

 we shall find that the ray, entering the 

 water at C, will pass on in the same 

 straight line to the point Q. Hence it 

 follows, that a ray of light falling per- 

 pendicularly upon a refracting surface 

 undergoes no refraction or change in 

 its direction. If the tube A C is now 

 placed in the position A C, and a ray of 

 light be made to pass through it, the 

 ray will not pass on in a straight line, 

 but will be bent or refracted at C into 

 the line C E, and strike the circle at 

 E. The angle AGP, which the ray 

 or tube forms with the perpendicular 

 P C Q, is called the angle of incidence; 

 and the angle E C Q, which the re- 

 fracted ray CE forms with the same 

 perpendicular, is called the angle of 

 refraction. Let us now take in a pair 

 of compasses the line E F, the shortest 

 B 2 



