DISTANCE AND SIZE 249 



the lack of need, resides in the plan of the eye itself and not in the dis- 

 tance of the object. 



The Why of Accommodation — The ideal human eye, in a state of 

 internal rest, is said to be emmetropic. By this is meant that parallel rays 

 of light, striking the cornea through air, are brought to a point focus on 

 the retina (Fig. 12, p. 27). It needs to be emphasized that the average 

 human eye is not emmetropic. Emmetropia is decidedly the exception, 

 not the rule. The most prominent student of mammalian refraction, 

 Lindsay Johnson, found a slight and beneficial degree of hypermetropia 

 in the eyes of primitive peoples, and considered that this might be the 

 truly normal human situation. 



In emmetropia, if we should gaze at the sun for a long moment, a tiny 

 hole would be burned through the retina in the region of the fovea — the 

 focus of those intolerably intense, parallel rays. If we set up an experi- 

 mental source of milder parallel rays in a dark-room, it can be at any 

 distance beyond twenty feet and will theoretically always have the same 

 brightness, for the same sized light-pencil (just filling the pupil) will be 

 brought to a point focus. 



But natural objects are not giving off parallelized beams of light. From 

 each point on the object, rays diverge away in many directions and only 

 a few of them are aimed at the pupil of a nearby eye. Unless these are 

 brought to a point focus in the layer of visual-cell outer segments, we 

 will not see that object-point as a point and cannot build up a sharp 

 retinal image of the object (Fig. 10, p. 25). The light by means of which 

 we see the object-point is thus a cone, its apex the object-point and its 

 base the pupil — or more accurately, that circle of cornea from the circum- 

 ference of which rays bent by the corneal surface can just enter the pupil. 



As the object approaches, the cone of light entering the eye from each 

 point on it becomes a shorter, stubbier cone with a much greater angle at 

 the apex, so that more of the light rays emanating from each point are 

 now caught by the pupil and the object looks brighter. But these rays are 

 now striking the corneal surface at lesser angles than before and if the 

 ray-bending (focusing) power of the eye remains unchanged, the point 

 at which they are brought together will move backward in the eye and 

 slip off the tips of the visual cells into insensitive pigment epithelium. 

 The eyeball being then too short, its vision becomes like that of a hyper- 

 metropic or far-sighted eye (Fig. 12), and the visual cells register only 

 blur-circles. To bring the crisp image forward into the visual-cell layer 



