250 ADAPTATIONS TO SPACE AND MOTION 



again, either the optical center of the eye must be moved forward, farther 

 from the retina, or else the ray-bending power of the dioptric apparatus 

 must be increased — by sharpening the curvature of the lens, the cornea, 

 or both. 



Both of these general methods of accommodation — by moving the lens 

 (Figs. 98 and 99) or by increasing its curvature (Fig. 100) — are in use 

 among various vertebrates. Through evolution, there has been a tendency 

 to abandon the first method for the second, simply because of greater 

 ease of making it mechanically precise and positive, rather than because 

 of any inherent optical superiority of the one method over the other. 



Accommodation is necessary, then, to keep a sharp image of an ap- 

 proaching or receding object within the thickness of the visual-cell layer. 

 The word as used by medical men refers only to the adjustment for ap- 

 proach or static nearness, but this application is hardly broad enough for 

 our purposes; for, in some vertebrates, the resting eye is myopic, making 

 a saving of muscular effort since the eye is used mostly at close range. 

 Parallel rays are then focused in front of the retina, and the lens must 

 be moved backward to adjust for a distant object — a 'negative' accom- 

 modation as compared with our own (Fig. 98). 



Obviously, the need for accommodation depends upon two things : the 

 amount of forward or backward shift of the image relative to a given 

 shift of the object, and the length of the visual cells. When an emme- 

 tropic human eye fixates an object at the horizon, its image falls some- 

 where in the layer of outer segments — presumably very close to their 

 inner ends. Now, that object can approach the emmetropic eye to a dis- 

 tance of only twenty feet without its image moving backward a distance 

 greater than the length of the outer segments — a tiny fraction of a mill- 

 imeter. The approach of the object is thus minified far more than are any 

 sidewise movements it may make. The image moves backward faster and 

 faster, however, as the object comes up; and when it comes within twenty 

 feet the lens must begin to sharpen its curvature to keep the optical image 

 coinciding with the photochemical image in the outer segment layer. 

 When we have sharpened the curvature of the lens as much as we can, 

 and the object is still clearly seen, it is said to be at our 'near point' — 

 which may be a few inches before our eyes if we are young, or beyond 

 comfortable arm's reach if we are middle-aged and 'presbyopic' When 

 we are very old and the lens is too hard to deform at all, the near point 

 has of course receded from us to the twenty-foot distance (Fig. 15, p. 35). 



