406 



THE SENSORIAL FUiNCTIONS. 



the point where the ray meets the new medium, 

 perpendicular to the refracting surface. Thus the 

 ray r, Fig. 408, striking obliquely on the surface 

 of a denser medium, at the point s, instead of pur- 

 suing its original course along the line s o, is re- 

 fracted, or turned in the direction s t, Mhich is a 

 line situated between s o, and s p ; this latter line 

 being drawn perpendicularly to the surface of the 

 medium, at the point s, and within that medium. 

 When the ray arrives at t, and meets the posterior 



surface of the dense medium, passing thence into 

 one that is less dense, it is again refracted accord- 

 ing to the same law; that is, it inclines towards 

 the perpendicular line t t, drawn from t, within 

 the denser medium, and describes the new course 

 T u instead of t v. The amount of the deflection 

 corresponds to the degree of obliquity of the ray to 

 the surface which refracts it ; and is mathematically 

 expressed by the law, that the sines of the two 

 angles formed with the perpendicular by the inci- 

 dent and the refracted rays retain, amidst all the 

 variations of those angles, the same constant pro- 

 portion to one another. We may hence derive a 

 simple rule for placing the plane of the refracting 

 surface so as to produce the particular refraction 

 we wish to obtain. When a ray is to be deflected 



