454 THE SENSORIAL FUNCTIONS. 



point s, and within that medium. When the 

 ray arrives at t, and meets the posterior sur- 



408 



face of the dense medium, passing thence into 

 one that is less dense, it is again refracted 

 according to the same law ; that is, it inclines 

 towards the perpendicular line t i, 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 re- 

 fracts it ; and is mathematically expressed, by 

 the law, that the sines of the two angles formed 

 with the perpendicular by the incident and the 

 refracted rays retain, amidst all the variations of 

 those angles, the same constant proportion 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 from 

 its original course to a particular side, we have 

 only to turn the surface of the medium in such 

 a manner as that the perpendicular line to that 



