DESCRIPTION OK PLATE XL. 



Vic., i. A diagram illustrating the course of the light rays in the compound eye 

 according to the Dioptric theory of vision : a, />, the object ; <z\ a", points from 

 which light radiates ; /', 1-, corneal facets ; i i, subcorneal images ; a\ i>\ retinal 

 image. 



Fie. 2. Diagrams illustrating the optical properties of refracting cylinders: 



A, A refracting cylinder, with several focal points ; //, parallel rays of light ; 

 a, />, <, d, the cylinder ; fff, succe:-sive foci ; n /!, >i ] w 1 , successive forms of the 

 wave front a, b ; r, /i, </, curved face of emergence ; gg, direction of rays leaving 

 the cylinder by its face r, d ; f, focus of the same rays after refraction at the sur- 

 face f, A, d. 



B, A diagram illustrating the optic properties of a refractive cylinder on the 

 principles of mechanical optics (see p. 572) : a a, b l>, successive forms of the 

 wave front ; .v, </, a 1 , path of a ray of light refracted at the surface y y. 



C, A refractive cone after Exner : a, l>, object ; /', a, subcorneal image ; 

 f 1 p\ "i } i l , circles of confusion due to the pencils//, /// in. 



D, A refractive cylinder on the Dioptric theory : /\ i, retinal image ; 

 r r, a, second refracting surface, causing a more rapid convergence of pencils of 

 light. 



. 3. A diagram illustrating the relative illumination of the retinal image under 

 the Mosaic and Dioptric theories (see p. 575). 



