414 THE OUTGO OF ENERGY 



Draw with ink a ray incident to the refract- 

 ing surface. 1 Find the sine of the angle of in- 

 cidence. 2 For the Frauenhofer line D passing 

 from air into crown glass the ratio of the sine 

 of the angle of incidence an to the sine of the 

 angle of refraction bn is 



an '. bn '.'. 1.53 '. 1 



For the same light passing from crown glass 

 into air the ratio of the sine of the angle of in- 

 cidence to the sine of the angle of refraction is 

 the reciprocal of the ratio from air to crown glass 



an '. bn '. '. 1 '. 1.53 



middle of the base line. Let a' point on this perpendicular 

 5 crn. above the base line be the apex of the section. Join the 

 apex with the ends of the base line. Ink the boundary lines of 

 the cross-section thus obtained. Erase the pencil construction 

 lines. 



1 For convenience let the incident ray come from the pro- 

 longed base line of the prism 10 cm. from the nearest refract- 

 ing surface. Let the point of incidence the point at whidh 

 the incident ray meets the refracting surface be about the 

 middle of the refracting surface. 



2 To find the sine: With the point of incidence as a centre 

 draw a circle of convenient radius (2 cm.). Construct a radius 

 of this circle perpendicular to the refracting surface at the point 

 of incidence. From the intersection of the circle with the in- 

 cident ray draw a line perpendicular to the radius (a line drawn 

 from the point of intersection parallel to the refracting surface 

 will be perpendicular to the radius). This is the sinus line of 

 the angle. The ratio of this line to the radius of the circle is 

 the sine of the incident angle. 



