istance of Luminous Point. 



5 m . 

 e m . 

 2 m . 

 5 m . 

 - m '. 

 i room. 

 o'.75om. 

 0375111. 

 o i88m. 

 0.094 m. 

 0.088 m. 



THE SENSE OF SIGHT 



Distance of the Focal 



Point behind the Posterior 



Surface of the Retina. 



o.o mm. 

 0.005 mm. 

 0.012 mm. 



695 



Diameter of the Diffusion-circle. 



0.050 mm. 

 o. 100 mm. 

 0.20 mm. 

 mm. 



mm ' 

 mm. 



mm. 

 mm. 



0.40 



i. 60 

 3-20 

 3-42 



o.o 



o.oon 



0.0027 



0.0050 



0.0112 



o.o222 



0.0443 



0.0825 



o.i6i6 



0.3122 



0.5768 



0.6484 



mm. 

 mm. 

 mm. 

 mm. 

 mm. 

 mm. 

 mm. 

 mm. 

 mm. 

 mm. 

 mm. 

 mm. 



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

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

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

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

 or their correction an accommodative effort which is scarcely measurable. 

 rom 6 meters up to 6 centimeters, however, a progressive increase in ace 

 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 

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

 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 

 point reflected from the anterior surface of the cornea and 

 the anterior and posterior surfaces of the ^ lens, change 

 their relative positions during accommodation. 



Thus, if in a darkened room a lighted candle be placed 

 in front of and to the side of an individual whose eye is 

 directed to a distant object, an observer placed in ^the 

 same relative position as the candle will observe tnree 

 images in the eye, one at the surface *****^ 

 two at the pupillary margin (Fig. 302). Of the two latter, 

 one is auite large and situated apparently in front ot the 

 small, and inverted. The midcUe 



of the lens> c . In _ 

 verted imag e, from 



tnira, wmcii is imiu, * . 



image is_reflected 6^JJ^jSMS 



rtw 



ccording to Helmholtz, during accommodation the entire antenor sur- 



