INTRODUCTION TO PHYSIOLOGICAL OPTICS 415 



Measure the length of the sine of the angle of 

 incidence in millimetres. Suppose that an in 

 the present instance is 13 mm. Then with 

 equation (1) 



1.53 : 1 : : 13 : x 



x = 8.5 mm., the sine of the angle of refraction. 



Find on the construction circle within the 

 prism a point 8.5 mm. in a perpendicular line 

 above the diameter at right angles to the refract- 

 ing surface. Continue the ray through this point 

 to the second refracting surface. Ink the path 

 of the ray within the prism. Erase the con- 

 struction lines within the prism, but leave un- 

 touched those without the prism. Find the sines 

 of the angles of incidence and refraction at the 

 second refracting surface. Draw with ink the 

 path of the emergent ray. Preserve all these 

 construction lines. Write the equations in ink 

 in the upper left-hand corner of the paper, and 

 the four sines in the upper right-hand corner. 



The degree to which light is refracted on pass- 

 ing through a prism depends on the refracting 

 power of the substance of the prism, the size of 

 the refracting angle, and the size of the angle of 

 incidence. 



