SENSE OF VISION. 



747 



trary, the rays from an object at a greater distance would meet before they 

 reach the retina, and would have again diverged from each other when they 

 impinge upon it; so that, in either case, vision would be indistinct. Now, 

 two methods of adaptation suggest themselves to the Optician. Either he 

 may vary the distance between the refracting surface and the screen on 

 which the image is formed, in such a manner that the latter shall always be 

 in the focus of the converging rays ; or, the distance of the screen remain- 

 ing the same, he may vary the convexity of his lens,. in such a manner as 

 to adapt it to the distance of the object. The mode in which this adapta- 

 tion is effected in the Human Eye has been carefully investigated by Czer- 

 mak, Helmholtz, and others. According to the calculations of Olbers, based 

 on the ascertained refractive powers of the media of the eye, the difference 

 between the focal distances of the images of two objects, the one so far off 

 that its rays are parallel, and the other at the distance of only four inches 

 from the eye, is about 0.143, or one-seventh of an inch ; but as the usual 

 range of distinct vision does not extend to objects brought within six or seven 

 inches, the amount of change required in the relative places of the refract- 

 ing bodies and the retina would not ordinarily exceed a line. It has been 

 thought that this change might be produced by an alteration in the con- 

 vexity in the cornea, or by an elongation of the globe of the eye generally, 

 or by both methods in combination ; but a strong counter-argument to these 

 opinions is derived from a case which came under the observation of v. Grafe, 

 in which the power of accommodation was preserved, although from paraly- 

 sis of the Third Nerve all the ocular muscles were paralyzed, except the 

 external rectus and the superior oblique. There is much more ground for 

 the belief that a change of form and perhaps of place is effected in the crys- 

 talline lens, by the action of the ciliary muscle and the erectile tissue of the 

 ciliary processes ; for not only can it be shown that the contraction of the 



FIG. 261. 



0, Cornea; s, Sclerotic; r c N, Vertical plane of the Cornea; BCD, Axis of the Eye; s s, Canal of 

 Schlemm : p, Angle formed by the Iris and Cornea, or margin of anterior Chamber ; m, Position of Iris, 

 and curvature of Lens in an Eye converged for parallel rays, distant vision, or negative accommoda- 

 tion; n, Position of Iris, and curvature of lens required for near objects, or for positive accommo- 

 dation. 



ciliary muscle would tend to compress the lens, but the fact that this muscle 

 is peculiarly powerful in the predaceous Birds, which are distinguished for 

 their great range of vision, and which have, in their circle of osseous scle- 

 rotic plates, an unusually firm point of attachment for it, is a strong argument 

 in favor of this doctrine. 1 Further, the almost entire loss of the power' of 

 adapting the eye to distances, which is experienced after the removal of the 

 Crystalline lens in the operation for Cataract, is a marked indication that 



1 See on this subject, Messrs. Todd and Bowman's Physiological Anatomy, vol. ii, 

 p. 27; and Dr. Clay Wallace on The Adjustment of the Eye to Distances, New 

 York, 1851. 



