10 



PRACTICAL HISTOLOGY. 



20. Refraction and Reflection of Light. The 

 action of a lens depends on the following principles. As 

 long as a luminous ray travels in a uniform medium it 

 pursues a straight line. If it pass at a right angle from 

 one medium to another, it does not deviate from its 

 course, however different the refractive powers of the two 

 media may be. But if the ray traverse the plane sepa- 

 rating two media of different refractive powers at any 



other than a right angle, it 

 is bent. Thus, the ray R 

 (Fig. 6), passing through air 

 and striking the surface of a 

 glass plate obliquely at B, 

 is bent in two directions ; a 

 portion (r) is reflected, while 

 the remainder is refracted 

 towards the perpendicular 

 (/). The refracted ray again 

 strikes the plane between the 

 two media obliquely at C ; a 

 portion (r) is reflected, while 

 the remainder is refracted 

 from the perpendicular as it re-enters the air. The emergent 

 ray is of less intensity after traversing the glass, in conse- 

 quence of its partial reflection at the 

 two surfaces ; there is therefore a loss 

 of light at every refraction ; a very im- 

 portant fact in microscopical optics. 



21. Index of Refraction. 

 The refractive power of any medium 

 is expressed by its index of refrac- 

 tion. This is ascertained by measur- 

 ing the amount of refraction which 

 a ray undergoes in passing from air 

 into another medium ; thus, let r 

 (Fig. 7) be a ray passing through air, 



incident upon water at /", and refracted in the direction r. 

 The sines of the angles of incidence r i p, and of refraction 

 r i /', are obtained by describing a circle with the point 



FIG. 6. Refraction and reflection by 

 parallel plate. 



FIG. 7. Law of refraction. 

 (Lorn me I.) 



