414 COURSE OP REFRACTED RAYS. 



is, that the rays of light, as they pass out of the water, are 

 bent downwards, or from the perpendicular ; so that they 

 reach the eye of the observer when situated at a lower point 

 than that at which the rays would have arrived if they had 

 proceeded in a straight line. Thus the eye, situated at the 

 end of the line a c, could not see the coin in a straight line, 

 because rays passing in that line would be interrupted by the 

 opaque sides of the basin ; but it receives the ray which was 

 passing through the water in the direction a d, and which 

 was bent downwards at the moment of quitting it. If the 

 eye had been placed directly over the coin, however, so that 

 the ray passing through the latter to it would have emerged 

 from the water in a direction perpendicular to its surface, no 

 change in the apparent place of the object would have been 

 made by pouring-in the water ; since a ray that passes from 

 one medium to another, however different, in a direction 

 perpendicular to the surface which separates them, is not 

 refracted. Those rays which pass-out most nearly in this 

 direction are refracted least, whilst those which pass-out most 

 nearly in the horizontal direction are refracted most. 



528. The general law of refraction then is, that all rays 

 passing from a dense to a rare medium are refracted from the 

 perpendicular, the degree of change being less as they are 

 near the perpendicular, and greater as they depart from it. 

 On the other hand, when rays pass from a rare medium into 

 a dense one, they are bent towards the perpendicular ; and 

 this in a greater or less degree, according as their direction is 

 more distant from the perpendicular, or nearer to it. Thus, 

 in fig. 205, we will suppose the point a to be the position of 

 the eye of a Fish ; and the end of the line a c (previously 

 occupied by the eye of the observer) to be the position of an 

 Insect in the air. Now this insect will not be seen by the 

 fish in its true place ; for a ray passing from it to c would be 

 so bent out of its course as not to reach the point a. The 

 direction in which it is really seen is a d ; for the ray pro- 

 ceeding from the object to the surface of the water, there 

 undergoes such a refraction that it is bent downwards to a ; 

 and, as we always judge of the place of an object by the 

 direction in which the rays last come to the eye, the insect is 

 seen by the fish at d, that is, considerably above its real place 

 ( 476). 



