REFLEXION AND REFRACTION. 23 



Let u and u\ denote the angles of incidence and refraction 

 at the first surface. On account of the parallelism of the 

 refracting surfaces, the angle of incidence on the second will 

 be equal to i, the angle of refraction at the first. Let u z he 

 the angle of refraction at the second surface. Then 



sin u sin u 2 



sin -u-i sin K I 



Therefore u t = n, or the incident and emergent rays are equally 

 inclined to the normals to the refracting surfaces. 



; 



(28) When a ray of light traversing a medium bounded 

 by parallel planes enters a third, which is different from either 

 of the former, its course in the third medium will be the 

 same as if the light had entered it directly from the first, and 

 is, therefore, independent of the intervening medium. 



This law is a consequence of the observed fact, that when 

 the light emerges from the third medium into the first, the 

 bounding surface being parallel to the former, the emergent 

 ray is parallel to the incident. Let another ray, parallel 

 to the former, be incident directly from the first into the 

 third medium, and traversing it, emerge again into the 

 first ; the emergent ray will be parallel to the incident, and 

 therefore parallel to that which has traversed the two media. 

 These two rays, consequently, will be equally refracted 

 at the second surface of the third medium, and therefore the 

 portions in the third medium will be also parallel. Hence, 

 when a ray passes from any medium, through an intervening 

 one, into a third, all being bounded by parallel surfaces, the 

 total deviation of the ray is the same as if it had passed 

 directly from the first into the third". 



Let u denote, as before, the angle of incidence on the first 

 surface, u\ and u 2 the angles of refraction at the first and 



