ADJUSTMENT OF EYE TO DISTANCE. 441 



When the eyes are turned from a distant to a near object, the anterior 

 erect, and the posterior inverted image, undergo no change of position, 

 but the middle erect and least luminous image advances somewhat 

 towards the anterior erect image ; at the same time, the pupillary 

 margin of the iris undergoes a slight forward inclination, approaching 

 the cornea. Unless the change just mentioned in the middle image 

 be owing to some other conditions, such as an alteration in the position 

 of the eyeball, or an increase in the convexity of the cornea, either 

 with, or without, a forward and backward movement of the lens, it 

 must be produced by some alteration in the form of the anterior sur- 

 face of the lens itself. No change in the position of the eyeball, how- 

 ever, is necessary in the act of accommodation ; the cornea presents 

 no change in its convexity, such as has been supposed to be produced 

 by the muscles external to the eyeball; any assumed movement of the 

 lens, as a whole, forwards and backwards, is inconsistent with the fact 

 that, according to the best observers, the posterior surface of the lens 

 does not shift its position. The phenomena actually observed can 

 therefore only be explained by supposing that an alteration occurs in 

 the convexity of the anterior surface of the lens. 



An instrument named the ophthalmometer has been used, to over- 

 come the difficulties of ascertaining and measuring the minute changes 

 in the relative shape and position of the two images of the flame re- 

 flected from the anterior and posterior surfaces of the lens, in the ex- 

 periment just referred to; and many most careful determinations of 

 the actual changes which take place in the lens, in the position of the 

 ciliary processes, and in the condition of the iris, have now been made. 

 An increased convexity of the anterior surface of the lens, a forward 

 movement of this surface, so that it approaches the cornea, and a 

 necessary increase in its antero-posterior axis, have thus been noticed. 

 The radius of its anterior surface is diminished, and its vertex ap- 

 proaches the cornea. In the normal eye, the radius of the curve of 

 the anterior surface of the lens is said to measure 8.8 millimetres in 

 distant vision, and 5.9 in near vision ; in the former case, the distance 

 of the anterior surface of the lens from the surface of the cornea is 3.9 ; 

 in the latter, 3.4 millimetres. The posterior surface of the lens is said 

 to undergo but little, or, according to the best authorities, no change 

 either in shape or position. By some, however, a slight forward 

 movement of the lens is supposed to occur in near vision. During 

 near vision, the pupil contracts, whilst the pupillary margin of the 

 iris moves forwards, and its attached border or rim falls backwards 

 (Helmholz). On this point, however, contrary statements have been 

 made ; for the plane of the whole iris is said by Knapp to move for- 

 wards j^th of an inch ; whilst, according to Czermak, it undergoes no 

 change, remaining perpendicular. It is, moreover, stated that, in 

 near vision, the points of the ciliary processes recede from the margin 

 of the lens (Becker). In accommodation for distant vision, on the 

 other hand, the pupil dilates, the inner border of the iris falls back, 

 and the points of the ciliary processes are said to approach the mar- 

 gins of the lens. 



The altered shape of the anterior surface of the lens, now generally 



