DESCRIPTION OF PLATE XL. 
Fic. 1.-—A diagram illustrating the course of the light rays in the compound eye 
according to the Dioptric theory of vision: a, 4, the object; a', a*, points from 
which light radiates ; 21, 2%, corneal facets ; 7 7, subcorneal images ; a’, 4!, retinal 
image. 
Fic. 2.—Diagrams illustrating the optical properties of refracting cylinders : 
A, A refracting cylinder, with several focal points ; A, parallel rays of light ; 
a, b, c, d, the cylinder ; f/f, successive foci; 7 7, n! n', successive forms of the 
wave front a, 4; c, 4, d, curved face of emergence ; gg, direction of rays leaving 
the cylinder by its face ¢, d; 7, focus of the same rays after refraction at the sur- 
face c, h, d. 
B, A diagram illustrating the optic properties of a refractive cylinder on the 
principles of mechanical optics (see p. 572): @4, 6 6, successive forms of the 
wave front ; x, g, x1, path of a ray of light refracted at the surface y y. 
C, A refractive cone after Exner: a, 4, object ; 6, a, subcorneal image ; 
?' p', m m', circles of confusion due to the pencils p A, 7 7. 
D, A refractive cylinder on the Dioptric theory: /', 7, retinal image ; 
yr, a, second refracting surface, causing a more rapid convergence of pencils of 
light. 
Fic. 3.—A diagram illustrating the relative illumination of the retinal image under 
the Mosaic and Dioptric theories (see p. 575). 
