776 



A MANUAL OF PHYSIOLOGY 



When a ray passes across a medium bounded by parallel planes, it 

 issues parallel to itself; in other words, it undergoes no refraction 

 (Fig. 279). 



Refraction and Dispersion by a Prism. The beam of light is bent 

 towards the normal N as it passes across BA and away from the 



FIG. 278. REFRACTION AT A PLANE FIG. 279. REFRACTION BY A 



MEDIUM BOUNDED BY PARALLEL 

 PLANES, P AND P'. 



The ray ABDE issues parallel to its 



SURFACE. 



AB is the incident; BD, the refracted 

 ray ; CB, the normal to the surface. When 



the ray passes from air into another medium, original direction ; CB, FD, normals to 

 the refractive index of the latter is the fraction P and P' ; a, angle of incidence ; ft, i, 

 sin * angles of refraction. 



normal N' as it passes across BC (Fig. 280) ; at both surfaces it is 

 bent towards the base of the prism AC. At the same time the light 

 suffers dispersion ; that is, the rays of shorter wave-length are more 

 refracted than those of greater wave-length. The deviation of any 



FIG. 280. REFRACTION AND DISPERSION BY A PRISM. 



given ray is measured by the angle which the refracted ray makes 

 with its original direction. The amount of dispersion produced by 

 a prism is measured by the difference in the deviation of the extreme 

 rays of the spectrum. The dispersion produced by any given sub- 



