THE SENSE OF SIGHT. 



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 indistinctness 

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

 the refraction and focusing of divergent rays (Fig. 291, P) parallel 

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

 again diverging, will form diffusion- circles. It is evident, there- 

 fore, that it is impossible to simultaneously focus both parallel 

 and divergent rays, and to see distinctly 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. The 

 capability which the eye possesses of adjusting itself to vision at 

 different distances is termed accommodation. 



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 : 



Distance of Luminous Point. 



oo 



65 m. 



25 



12 



6 



3 

 1.500 



0.750 



-375 

 o.i 88 



0.094 

 0.088 



Distance of the Focal 



Point behind the Posterior 



Surface of the Retina. 



o.o mm. 

 0.005 



O.O 1 2 



O.025 



0.05O 



O.IOO 



0.20 



0.40 



0.80 



1. 60 



3.20 



3-42 



Diameter of the Diffusion-circle, 

 o.o mm. 

 o.oon 

 0.0027 

 0.0056 



O.OII2 

 0.0222 

 0.0443 

 0.0825 



0.1616 

 0.3122 

 0.5768 

 0.6484 



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 nearness of the object toward 

 which the eye is directed. 



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 centi- 

 meters, however, a progressive increase in accommodative power 

 is demanded for distinct vision. 



Mechanism of Accommodation. Inasmuch as neither the 

 corneal curvature nor the shape of the eyeball undergoes any change 



