272 



EYE-SIGHT, ITS DEFECTS AND TREATMENT. 



obscure or blurred outskirts. The eye sweeps 

 IN glance over a wide range, and forms clear 

 pictures of every part of the field in such rapid 

 succession as to receive the impression as a 

 whole. This effect is, no doubt, promoted by 

 the curious anomaly that, while the vibrations 

 of light pass through a distance of 200,000 miles 

 ,in a second, the nerve-intluence passes from the 

 eye to the bruin at the comparatively sluggish 

 r.ite of only two hundred feet in a second. 



But the most important difference between 

 the eye and the camera, and one that has only 

 recently been fully understood, is to be found 

 in the means employed for adjusting the instru- 

 ment to the reception of light from near and 

 remote objects. The lenses of the camera have 

 each a constant focal length ; that is, they bring 

 parallel rays together at a certain distance from 

 their own plane. If the rays are divergent 

 when received, a part of the power of the lens 

 is used up in bringing them parallel, and it 

 must be farther from the screen, in order to 

 concentrate the image distinctly upon it. So, 

 if they are partly converged already, the lens 

 must be nearer the screen. Hence, by moving 

 the lens forward or back, the adjustment to 

 near or distant objects is made. It was former- 

 ly supposed that the eye had some such power 

 of elongating and compressing itself along the 

 axis of vision, but this has been entirely dis- 

 proved. Its mode of adjustment is entirely 

 different. The eyeball is a sphere kept in 

 shape by the fluids contained within its strong 

 walls. These fluids serve a further purpose, 

 not completely understood, in connection with 

 the lens, in refracting the rays of light, but the 

 lens alone contains the power of adjustment 

 to distances. It is well established that there 

 is a constant adjustment of the power of vision 

 to the distance of objects. When the sight is 

 fixed upon near objects, distant ones in the 

 same direction can not be distinctly seen, and 

 vice versa. Donders, the Dutch physiologist, 

 furnishes a simple but effective illustration of 

 this peculiarity of eye-sight. If a net is held 

 between the eyes and a printed page, the at- 

 tention may be fixed on either the net or the 

 page, and either can be distinctly seen, but not 

 both at once. The attention may be rapidly 

 transferred from one to the other, and the proc- 

 ess of adjustment will follow it. The change 

 which takes place is wholly in the form of the 

 crystalline lens, not in its position or the shape 

 of the eye. This lens is suspended back of the 

 iris and between the aqueous and vitreous hu- 

 mors. It is contained in a sort of sheath or 

 shell of a slightly horny texture, which is set 

 in a circular rim. This vim is stretched out 

 in all directions and held firmly in place by 

 seventy radiating elastic bands. These bands 

 are about one fifth of an inch in length, and 

 are attached at their extremities to the outer 

 coat of the eye. They stretch the lens to its 

 fullest circumference, maintaining a certain flat- 

 ness which adapts it to the light from distant 

 objects, so as to bring it to a perfect focus 



on the retina. .Connected with these elastic 

 bands is a series of muscular fibers acting 

 antagonistically to their elasticity ; that is, 

 when the muscles contract, the bands relax and 

 allow the lens to bulge to a greater convexity, 

 it having a natural tendency to do so when not 

 stretched out. When the muscular libers are 

 brought into action so as to increase t.ie eim- 

 vexity of the lens, the vision is adapted to near 

 objects; and it is the constant play of the elas- 

 tic bands and muscular fibers, together with 

 the ready movement of the eye, that adapts the 

 sight to objects at all ranges of distance and di- 

 rection. The expansion and contraction of the 

 iris, so as to admit more or less light at once, 

 add effectiveness to the marvelous contrivance 

 by which the impressions of the external world 

 are conveyed to the brain. It is manifest that 

 the eye must be in constant action during the 

 hours of wakefulness, and that its delicate 

 mechanism is subjected to a strain from which 

 the only absolute rest is sleep or complete 

 darkness. 



These conclusions regarding the method by 

 which the power of vision is adjusted to vari- 

 ous distances have been thoroughly established 

 by the investigations of Sanson, Langenbeok, 

 Cramer, and Helmholtz, and are adopted by H. 

 Brudenell Carter, one of the latest authorities 

 on eye-sight. Professor Helmholtz has con- 

 trived an instrument for observing and measur- 

 ing the various curvatures of the crystalline 

 lens, which is turned to practical account in 

 producing optical compensations for defective 

 sight. This perfected knowledge of the struct- 

 ure of the eye has in fact had an important 

 effect upon the treatment of defective vision. 

 The crystalline lens is constructed out of a 

 series of flattened fibers of albuminous sub- 

 stance, grouped in symmetrical loops around 

 six separate axes, and connected together at 

 their edges by interlocking teeth. The trans- 

 parent mass is soft and pliable in youth, but it 

 gradually grows denser with advancing years. 

 This interferes with the facility with which it 

 can be adapted to near vision, and is the cause 

 of failing eye-sight. In early life the lens can 

 be curved easily so as to bring objects not 

 more than four and a half inches from the eye 

 clearly into view, but at the age of forty it 

 can not form a clear picture of objects nearer 

 than nine inches. At fifty the point of nearest 

 sight is removed to thirteen inches, at sixty to 

 twenty-six inches, and usually at seventy all 

 power of accommodation is practically gone. 

 The lens retains the uniform contour adapted 

 to distant objects, while it transmits the light 

 with a somewhat impaired degree of perfec- 

 tion. The remedy for this is to supply the 

 needed power of refraction by artificial lenses 

 placed before the eye. By this means the 

 power of clear vision for a certain fixed but 

 convenient minimum of distance is completely 

 restored. The artificial lenses should vary in 

 convexity according to the needs of the eye. 

 Mr. Brudenell Carter dwells with considerable 



