BIOLOGY. 239 



seemed to be all that was needed to show that the explanation 

 was true. It certainly had the merit of meeting the facts, and so 

 has been almost universally accepted by physiologists, and has 

 found its way into every text-book touching upon the optical struct- 

 ure of the eye. That these conditions, if they existed, would 

 produce the effects indicated, no one will doubt; but it should not 

 be lost si"ht of that the alleged conditions of the cornea and lens 



^j ^3 



were never satisfactorily shown to be attendants of the two ab- 

 normal states of the eye of which we are speaking. Recent 

 investigations have proved that both near and long-sightedness 

 may be, and in most cases are, the result of wholly other causes. 



A moment's reflection will make it apparent to any one that, the 

 refracting media being quite normal, if, in consequence of the axis 

 of the eye being too long, the retina is too far behind the^lens, the 

 rays will meet in front of this, and thus short-sightedness will of 

 necessity follow. The average length of the axis of the eye is a 

 little less than an inch, namely, 24.25 millimetres, or about 0.95 

 inch. Bonders has shown that in near-sighted persons it exceeds 

 an inch, and may amount to 1.2 inch, and even more, the other di- 

 ameters being unchanged. In this case the ball of the eye be- 

 comes more or less oval or egg-shaped, and when turned strongly 

 toward the nose will fill the orbit more than usual at the outer angle. 

 Concave glasses will, of course, be required to disperse the light 

 sufficiently to bring the rays to a focus on the retina. In proof 

 that too great convexity of the cornea does. not produce near- 

 sightedness may be urged the fact that this convexity is greatest 

 in children, but, as Volkrnann observed, children are rarely near- 

 sighted. 



In regard to long-sightedness, if the alleged cause of it, namely, 

 the flattening of the cornea and crystalline lens, existed, this 

 would of necessity form the focus, other things being the same, 

 behind the retina ; but no proof Was ever brought forward that 

 this flattening actually did exist in the majority of cases. In 

 adopting this explanation, its inconsistency with the fact that el- 

 derly persons still see far objects distinctly seems to have been 

 overlooked by physiologists. The persistence of this faculty was 

 of itself sufficient evidence to make it probable that no permanent 

 change took place in the form of the lens, since this would impair 

 the eye for seeing objects at a distance, as well as those near at 

 hand. Kramer and Helmholz have shown that the accommoda- 

 tion of the eye for seeing near objects depends upon a temporary 

 change in the form of the lens, this becoming more and more con- 

 vex as the object approaches the nearest point of distinct vision. 

 This is proved by watching the relative position of the three 

 images of a candle as seen reflected, 1, from the front of the cor- 

 nea ; 2, from the foremost or convex surface of the capsule of the 

 lens ; and 3, from the hindmost or concave surface of this cap- 

 sule. The image from this last is inverted, and that from the 

 front of the capsule is in the middle of the three. 



The attention of the person whose eye is observed being di- 

 rected to a distant point, if it be suddenly changed to a near one 

 in the same straight line with the first, so that no motion of the 



