7IO TEXT-BOOK OF PHYSIOLOGY 



Distance of the Focal 



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



Surface of the Retina. 



co o.o mm. o.o mm. 



65 m. o.oosmm. 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.75om. 0.40 mm. 0.0825 mm. 



0.375 m. 0.80 mm. o.i6i6mm. 



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



o.o94m. 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 

 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 

 FIG. 299. CATOP- e e , , ., .. * 



TRIG IMAGES ix THE m front of and to the side of an individual whose eye is 

 EYE. a. Upright directed to a distant object, an observer placed in the 

 fmTthfco^a! 10 ^ ? ame re . lative position as the candle will observe three 

 upright image from images in the eye, one at the surface of the cornea and 

 of 6 the te ien s r ^irT tw ^ the P u P illar y m argin (Fig. 299). Of the two latter, 

 verted 6 image, 'from one * s ^u^ 6 large and situated apparently in front of the 

 the posterior surface third, which is faint, small, and inverted. The middle 

 lens *~~ (Ife/m ~ image is reflected from the anterior surface of the lens, 

 the last from the posterior surface. These images of re- 

 flection are known as catoptric images. If now the individual be directed to 

 fix the gaze on a near object, the second image changes its position, ad- 

 vances toward the corneal image and at the same time becomes smaller, 

 a change which, in accordance with the laws of optics, could only be due to 

 an increase in the convexity of the anterior surface of the lens. A slight 

 displacement of the third image sometimes observed indicates a possible 

 increase in the convexity of the posterior surface of the lens. 



According to Helmholtz, during accommodation the entire anterior sur- 



