252 



ADAPTATIONS TO SPACE AND MOTION 



There is nothing mystical about this twenty-foot distance. It is that 

 distance because of the size and structure of the human eye and the 

 length of human visual cells. This is most important to remember; for 

 there is a naive tendency for some to assume, on learning that some ani- 

 mals have no accommodation, that those animals must have hazy images 

 of all objects nearer to them than twenty feet. This is not true — the 

 twenty-foot distance is just as much a part of the human eye, and only 

 of the human eye, as its diameter or its weight. 



With Eye At Rest: 



With Full Accommodation; 



receptive (visual-cell) layer 



commencement point is at 

 infinity, since even parallel 

 rays focus behiind the retina 



(accommodation is used whien object is 

 anywtiere beyond near point) 



near point 



tar point is at infinity (parallel rays focus 

 at /nner surface of receptive layer) 



commencement point 



(accommodation is used over thus range 

 of object-positions) — n 



c.p. 



near point 



for point 



commencement point 



(accommodation is used over this range 

 of object-positions)| 



h-^-H 



c.p. 



near point 



Fig. 101 — Object-positions in relation to accommodation and refractive errors. 



The greatest distance at which an object can stand from an eye, and 

 still be sharply imaged on the retina, is called that eye's 'far point'. Sup- 

 pose we call by the name 'commencement point' that least distance an 

 object can have without there being any necessity of accommodation. 

 For the emmetropic human eye, then, the commencement point — the 

 point, in the approach of the object, at which the eye must begin to exert 

 accommodatory effort — is around twenty feet. The hypermetropic or far- 

 sighted human eye has a much more distant commencement point. Theo- 



