414 
COURSE OF KEFRACTED 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. Flow 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 ad; 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). 
