30 A TYPICAL VERTEBRATE EYE: THE HUMAN 



The ray now travels to the retina, having crossed the optic axis of the 

 eye so that it strikes the retina on the opposite side of the axis to the 

 one on which it entered the cornea. The retinal image of an object is 

 consequently inverted and much smaller than the object, as is true of 

 the image of any simple convex lens, as we have seen. The refractive 

 index of the human retinal tissue, which in life is optically empty, is not 

 known; but it may be of considerable importance in connection with the 

 physiology of the fovea (Chapter 8, Section C) . There are indications 

 that it is higher than that of the vitreous and may approach that of the lens. 



It should be borne in mind that it is the difference in refractive index 

 on the two sides of a boundary surface which, together with the sharp- 

 ness of curvature of that surface and the direction of curvature (whether 

 convex or concave), determines the extent of convergence or divergence 

 of light rays passing through it. The absolute values of the refractive 

 indices are of no consequence. Hence since the anterior surface of the 

 cornea is an interface between two very different media (air and tissue) 

 it is the most important refractive surface in the dioptric media. The 

 posterior surface of the lens is next in importance, the anterior surface 

 of the lens least effective (when the eye is not accommodating), and 

 the posterior surface of the cornea can be ignored entirely. 



It is the anterior surface of the lens, however, which in the human eye 

 is alone modified in curvature in the act of accommodation — hence for 

 this process, that surface is of paramount importance. We are now pre- 

 pared to examine the mechanism by which human accommodation is 

 accomplished. 



Accommodation — In the first place the reason for accommodation, 

 and the extent of the process, need to be clearly understood. The curva- 

 tures of the refractive surfaces of the ideal human eye and the refractive 

 indices of ordinary air and of the dioptric media are such that when the 

 eye is at rest — that is, exercising no muscular effort to accommodate 

 for nearby objects — objects at the horizon are in focus upon the 

 back surface of the transparent retina. The seeing-cells, the rods and 

 cones, stand on this surface like the bristles of a brush. Their length is 

 appreciable, and since a light ray which helps to form the image strikes 

 the retina perpendicular to its surface and thus passes axially through a 

 visual cell, it follows that the optical image may lie anywhere along the 

 length of the visual elements and still form the same photochemical 

 image, and be as sharply 'seen' in the form of a cerebral or mental image. 



