458 REFRACTION THROUGH PRISMS. 



tion but somewhat displaced; an eye at d sees the 

 object from which the ray a b proceeded as if the object 

 were at e. This lateral displacement is so slight that 

 it is only rarely perceived, viz. when the glass is of 

 unusual thickness. But if the glass has a suitable form, 

 a ray of light is found to be more bent from its original 

 path when passing from air into glass, than when it 

 passes from air into water; glass is hence said to have 

 greater ' refractive power ' than water. If one of two 

 rays is incident upon glass and the other upon water, 

 and the angle of incidence is the same for both rays, the 

 angle of refraction will be less in glass than it is in water. 



If a ray of light passes from glass into water, as in 

 Fig. 260, or from water into glass, the angle which the 

 ray makes with the perpendicular to the surface, will 

 always be less in glass than in water; in this as in all 

 other cases the law holds good, that of two different 

 substances that one in which the angle between the ray 

 and the perpendicular at the point of incidence (or emer- 

 gence) is the smaller has more refracting power than 

 the other. It follows from the preceding experiments 

 that air possesses less refractive power than water. 



A body possessing the shape represented in fig. 6 

 (page 6) is called a ' triangular prism/ Prisms of this 

 form made of transparent substances are very well 

 adapted for studying the effects of refraction. Prisms 

 for optical purposes require, however, only two ' re- 

 fracting surfaces,' that is, two plane faces which are 

 not parallel. Hence a transparent body of any shape 

 provided it has two surfaces inclined to one another, is 

 an ' optical prism/ The edge at which the two refract- 

 ing surfaces meet is the ' refracting edge ' or simply 



