852 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



By the term " refractive index " is meant the number which shows 

 how many times the sine of the angle of incidence (a 6, in Fig. 370, 

 regarding S D as the incident ray) is greater than the sine of the angle 

 of refraction (c d), it being always assumed that in comparing the 

 refractive indices of two media the incident ray passes from air into the 

 medium. On passing from air into water the ray is so refracted that the 

 sine of the angle of incidence is to the sine of the angle of refraction as 

 4:3; with glass, the proportion is 3 : 2. 



Fro. 371. DIAGRAM ILLUSTRATING THE COMPOSITION OF A CONVEX LENS OF 

 A NUMBER OF PLANE SURFACES. (Ganot.) 



An illustration, first suggested by Professor Henry Morton, of Hoboken, 

 serves greatly to simplify the conception of refraction. It has been stated that in 

 passing from a rarer to a denser medium the luminous ray is bent toward the 

 perpendicular. If a line of men, as of soldiers, be marching obliquely toward the 

 edge of a plowed field, the men first reaching tbe uneven ground will experience 

 difficulty in walking over the rough surface and their end of the line will move 

 more slowly than the end still remaining on the level ground, and, as a conse- 

 quence, the entire direction of the line of men will be changed. On the other 



FIG. 372. DIAGRAM SHOWING REFRACTION BY A DOUBLE CONVEX LENS. 



(Ganot.) 



The incident ray, L B, is refracted at the points of incidence. B, and emergence, D, toward the axis, 

 M N A, which it cuts at F. 



hand, as they reach the opposite side of the plowed surface the end of the line 

 which first entered will be the first to emerge, and, as a consequence, progress 

 now being easier, that end of the line will travel faster than the end still remain- 

 ing on the plowed ground, and, therefore, the line of men will now be bent from 

 ihe perpendicular; and as the entire line emerges the line of progress will be 

 parallel with the original line, but will not be coincident with it. 



