246 LAWS OF REFLECTION AND REFRACTION. 



makes it very easy in all cases to see in what direction tlie 

 ray always really bends in passing out of a rare medium 

 into a denser one. 



In the converse case, namely, that of a ray passing from 

 a dense medium into a rare one, the effect is exactly the 

 converse that is, a ray coming in the line R o, into v>- hich 

 the incident ray I o had been refracted, will, at the instant 

 of its issuing from the water, o, be held back, as it were, by 

 the superior influence over it of the greater mass of water 

 on the side W that is near enough to act upon it at the 

 moment of emerging, than by that on the side w, and so 

 will be drawn down into the direction o I, which is precise- 

 ly the same as that of the incident ray. Thus the action 

 of the water on entering and departing rays is equal and 

 reciprocal. 



This mode of stating the case is very indefinite and 

 vague, and would be wholly unsatisfactory considered in a 

 scientific point of view. It helps us very much, however, 

 in fixing in our minds the general law, to consider that, in 

 passing from a rare to a dense medium, the ray is bent to 

 ward the side where the mass of the dense medium lies 

 nearest to the course it is following. 



Stated scientifically, however, the law is, that the ray, in 

 passing from a rare to a dense medium, is bent toward the 

 perpendicular drawn from the point at which it enters. In 

 passing from a dense to a rare one, it is bent/rom the per- 

 pendicular to precisely the same extent. 



The diagram already referred to shows this very clearly. 

 The dotted line pp represents the perpendicular; lo is the 

 incident ray, entering the water at o. Instead of going on 

 in a straight course, as represented by the dotted line o v, 

 it is bent toward the perpendicular into the line o R. A 

 ray transmitted in the contrary direction that is, from R 

 to o, instead of continuing, when it emerges from the water, 



