12 VEGETABLE HISTOLOGY. 



is expressed by the Law of Refraction. The law is expressed 

 mathematically thus: 



sine of angle of incidence 



- = n, a constant, 

 sine of angle of refraction 



This ratio, n, is called the index of refraction, and is differ- 

 ent for the various transparent substances, but fixed for each 

 one. It may be accurately determined by physical experiments, 

 and is usually taken with reference to air as standard. There 

 is only one position in which light is not bent out of its path 

 in passing from air into any other medium, namely, when the 

 light rays are perpedicular to the surface of separation of the 

 two media. The index of refraction of water and glass are 

 n = 1.33 water, n = 1.52 glass (crown). Substances for which 

 n is greater than unity are said to be more refracting than air. 

 The refractive index has reference to light of one color only, 

 i. e., to monochromatic light. The yellow light obtained by 

 holding salt in a Bunsen flame is usually employed. 



When light passes from a denser medium into air the figure 

 and discussion is just the reverse of the above. A familiar 

 illustration of refraction is shown by dipping a stick obliquely 

 in water, when it will appear bent. The well-known fact that 

 an object under water is not in the place where it seems to be is 

 also explained by refraction. 



REFRACTION BY A PRISM. In optics a prism is any trans- 

 parent medium comprised between two plane faces inclined to 

 each other. The intersection of the two plane faces is called 

 the edge and their inclination is called the refracting angle. 

 Triangular glass prisms are generally used. Every section per- 

 pendicular to the edge is called a principal section, which is a 

 triangle in the case of a triangular prism. Fig. 6 represents 

 such a section, A is called the summit and BC the base of the 

 triangle. Let OI be a ray of light falling 

 upon the prism in the plane of the section 

 ABC. It will be refracted twice accord- 

 ing to the law of refraction, first as it 

 enters the prism at I, then as it leaves the 

 prism at D, so that the direction of the ray 



after emerging from the prism will be DS, 



instead of the original direction OIP. pig 6 



The amount of this deviation depends on 

 the size of the angle of the prism, its material and the angle of 

 incidence of the ray. The angle PRS measures the deviation 

 of the refracted ray from the original path and is called the 

 angle of deviation. It is seen that a ray of light is deflected 

 towards the base of the section ABC, hence its source O ap- 

 pears to be elevated or deflected towards the summit A. 



