INTRODUCTION TO PHYSIOLOGICAL OPTICS 413 



into crown glass, of which spectacle lenses are 

 made, is 1.530; into flint glass, 1.635; into water 

 at 15 C., 1.332. 



KEFRACTION BY PRISMS 



A refracting medium bounded by two plane 

 surfaces not parallel is termed a prism. The 

 planes are termed the refracting surfaces. The 

 angle which they make with each other is termed 

 the refracting angle of the prism. 



1. Place a prism in the optical box in the 

 beam of parallel rays. The beam will be bent 

 from its course on entering and on leaving the 

 prism. The emerging pencil will be divergent, 

 for the homogeneous rays, the union of which 

 produces the sensation of white light, are not 

 equally refracted, the rays towards the red end 

 of the spectrum are bent less strongly than those 

 towards the violet end, the order being red, 

 orange, yellow, green, blue, violet. 



Construction of the Path of a Ray passing through 

 a Prism. Draw a horizontal line 5 cm. in length. 

 Upon this line construct the section of a prisrn. 1 



1 To construct the section of a prism: Let the horizon- 

 tal line be the base of the prism. Place the brass leg of the 

 drawing compasses at one end of the base line. Draw a circle 

 of 3 cm. radius. Place the brass leg at the other end of the 

 base line and draw a circle of the same radius. A line joining 

 the intersections of the two circles will be perpendicular to the 



