658 TEXT-BOOK OF PHYSIOLOGY. 



from the distant to the near object, of a change in the eye itself, a change 

 that involves time and effort. The reasons for these facts will become 

 apparent from a consideration of the following facts: 



In a normal or emmetropic eye, parallel rays of light (Fig. 318, a, b) 

 after passing through the optic media are converged and brought to a 

 focus on the retina, /. Rays, however, which come from a luminous 

 point situated near the eye, P, and are therefore divergent, passing through 

 the optic media at the same time, are intercepted by the retina before they 

 are focused, and give rise to the formation of diffusion-circles and indis- 

 tinctness -of vision. The reverse is also true. When the eye is adjusted 

 for the refraction and focusing of divergent rays (Fig. 318, P) parallel rays 

 will be brought to a focus before reaching the retina, and, again diverging, 

 will form diffusion-circles. It is evident, therefore, that it is impossible 

 to focus simultaneously both parallel and divergent rays, and to see dis- 

 tinctly at the same time, two objects which are situated at different distances. 

 The eye must be alternately adjusted first to one object and then to another. 

 To this adjustment the term accommodation has been given. 



The following table of Listing shows the size of the diffusion-circles 

 formed of objects situated at different distances when the accommodative 

 power is suspended in an emmetropic eye : , 



Distance of the Focal 



Distance of Luminous Point. Point behind the Posterior Diameter of the Diffusion-circle. 



Surface of the Retina. 



o.o ram. o.o mm. 



65 m. 0-005 mm - o.oon mm. 



25 m. 0.012 mm. 0.0027 mm. 



.12 m. 0.025 mm. 0.0050 mm. 



6 m. 0.050 mm. 0.0112 mm. 



3 m. o.ioomm. 0.0222 mm. 



i.5Oom. 0.20 mm. 0.0443 mm. 



o.ysom. 0.40 mm. 0.0825 mm. 



0.375111. 0.80 mm. o.i6i6mm. 



o.i88m. i. 60 mm. 0.3122 mm. 



o.094m. 3.20 mm. 0.5768 mm. 



o.o88m. 3.42 mm. 0.6484 mm. 



From the foregoing table it is evident that between infinity and 65 meters, 

 the diffusion-circles are so slight that no perceptible accommodative effort 

 is required to eliminate them. From 65 meters to 6 meters the diffusion- 

 circles gradually become larger, though they are yet so faint as to require 

 for their correction an accommodative effort which is scarcely measurable. 

 From 6 meters up to 6 centimeters, however, a progressive increase in accom- 

 modative power is demanded for distinct vision. 



The normal eye when adjusted for distant vision is in a passive condition, 

 and hence vision of distant objects is unattended with fatigue. In the act 

 of adjustment, however, for near vision the eye passes into an active state, 

 the result of a muscle effort, the energy of which is proportional to the near- 

 ness of the object toward which the eye is directed. 



Mechanism of Accommodation. Inasmuch as neither the corneal 

 curvature nor the shape of the eyeball undergoes any change during accom- 

 modation, the necessary change, whatever it may be, is to be sought for in 

 the lens. As to the character of the changes in this body, two views are held, 

 based largely on the fact and its interpretation, that images of a luminous 



