Curvature Method of Teaching Optics. 475 



Thick Prisms. — Here we have only to take a section of 

 the prism ABC (fig. 2), and indicate the direction of the 



B' ■ C 



Refraction by Thick Prism. 



three wave-fronts before incidence, in the glass, and after 

 emergence by DE, DA, and DF respectively. The angles 

 O and (£,, are obviously the angles of incidence and of refrac- 

 tion with reference to the left face of the prism, while 2 and 

 (p 2 are those relating to the right face. We thus have imme- 

 diately z = <fi n + <p 2 and 8 + a, = 6 + 6 2 , and sinee sin 6 = /j,<m cf> 

 and sin 6 2 = /* sin <b 2 we have the distances DE, DF, and DA 



Fig-. 3. 



I 



V' 



i * i 



Refraction by Thin Lena. 

 in the ratio of 1 : 1 : fi, and we are immediately led to the 

 ordinary construction for, and properties of thick prisms. 

 Thin Spherical Lenses. — In tig. 3 we have a thin lens, the 

 ■1 \ -1 



