174 ELEMENTARY GENERAL SCIENCE CHAP. 



called its faces, which intersect in a line termed the edge of the 

 prism." It may also be denned as a solid having three rectan- 

 gular sides (Fig, 86). 



EXPT. 171. Focus on the screen a small circular hole in the 

 cap of the lantern. Introduce a wedge of glass, and show that 

 the spot is moved towards the base of the wedge. Let the 

 first face of the wedge be oblique to the beam. Put a second 

 wedge with the first, face to face and base to base, so as to 

 form a single wedge of double the angle. Notice the increased 

 displacement of the light spot. Put them edge to base, notice 

 the emergent beam is parallel to the incident. 



The amount of deviation of the beam thus depends upon the 

 angle of the wedge or prism ; it also depends upon the material 

 of which the prism is made and the nature of the incident 

 light. 



Path of a Bay of Light through a Prism. In Fig. 87 let 



FIG. 86. A Prism. FIG. 87. Path of a Ray of Light through a Prism. 



the triangle abc represent a section of the prism at right angles 

 to its faces, such as we should see by looking at the end of 

 it. Suppose DE is a ray of light striking the face ab of the prism. 

 The light on entering the prism passes from the air into the 

 glass, or from a rarer into a denser medium, and is, as was seen 

 in Expt. 166, bent towards a line drawn perpendicular to the 

 face of the prism at the point where the ray of light strikes 

 it. It consequently travels along the line EE' until it reaches 

 the face ac of the prism. Here it passes from the glass into the 

 air, i.e., from a denser into a rarer medium, and is, in such 

 circumstances, bent from the perpendicular, and travels along 

 the line E'D' . In every such passage through a prism it is 



