March 1, 1921 



THE INDIA RUBBER WORLD 



413 



thereby causing the lens, which is elastic, to become more or 

 less convex, and tlius varying its focal length. In this respect 

 the e>'e differs from the camera in which the focussing is done 

 by moving the lens back and forth. 



The inner surface of the eye, extending from the muscular 

 lens-mounting over the rear portion, is covered with a layer of 

 tissue consisting largely of nerve fibers, and is called the retina 

 R. A bundle of nerves — vcr>' like a large cable containing hun- 

 dreds of telephone wires — enters the eye-ball at the rear, and 

 slightly to one side B. These nerves spread out in all directions 

 over the inner surface, which is also well supplied with blood 

 vessels. The portion of the retina directly back of the lens, upon 

 which the image formed by the lens is received, is fitted with 

 the special devices that are directly acted upon by light. These 

 are of two kinds: small rods which project from the surface, 

 like the nap on velvet, and shorter cone-shaped projections, 

 pointing outward. The central part of this sensitive surface 

 is covered with cone.s only and the outer portion with rods only, 

 the two being interspersed in the median portion. These rods 

 and cones are the terminals of nerves. In the very center of this 

 sensitive surface there is a small spot, slightly depressed, and 

 of yellow color, called the fovea F, which is more highly sensitive 

 than the other parts of the surface, and upon which the eye 

 automatically throws the image of what we want to see with 

 particular sharpness. 



This sensitive portion of the retina is the counterpart of the 

 sensitive coating of the plate or film in photography. The cones 

 alone give the sensations we call colors ; while the rods alone 

 produce vision in very dim light, but give only sensations of 

 varying shades of gray. Who would have looked for so intricate 

 an explanation of the old saying that "in the night all cats are 

 gray" ! A little space where the nerve-cable enters the eye is 

 wholly insensitive to light, and forms the blind spot. Just what 

 action the light produces upon the rods and cones of the retina 

 which causes them to send nerve currents to the brain is not 

 known — another similarity to photography, the action of light 

 upon the silver salts in the film not being positively known. It 

 had been observed that, left in darkness or dim light, the rods 

 become purple at the ends, and that this purple rapidly fades 

 out on e.xposure to bright light. This indicates chemical action, 

 but further than this there is no definite knowledge. 



All of the effects of light which make up the sensation of 

 seeing are transmitted through the nerve-cable, or optic nerve, 

 to their special center — like a telephone "central" — in the brain. 

 The brain, like the eye, is a double apparatus, consisting of 

 duplicate right and left portions. It is a rather curious fact that 

 the optic nerves cross, the nerve from the right eye going to the 

 left lobe of the brain, and vice versa. If the optic nerve is 

 incapacitated, vision of course is prevented. Wood alcohol, taken 

 into the system either by drinking or by sufl'icient inhalation, has 

 the peculiar property of permanently paralyzing the optic nerve, 

 and so producing blindness — which renders it rather objectional 

 for beverage purposes. 



The interior cavity of the eye back of the lens is filled with 

 a transparent substance of buttery consistency called the vitreous 

 humor V, which serves to keep the casings distended and the 

 form full and true. 



The skin of the face is slit and folded under in front of the 

 eye-sockets, forming the eyelids, which serve as the shutter for 

 the camera. It is important to remember, however, that the 

 eyelids are not opaque, like the shutter in the photographic 

 camera, but decidedly translucent, as may be readily seen by 

 closing them in the light and observing the ver>' sensible im- 

 pression of light still produced upon the mind. 



GENERAL CONDITIONS FOR GOOD VISION 



Having made ourselves familiar with the general construction 

 of our seeing apparatus, we may now inquire into the conditions 

 necessary for obtaining the best results from its use. To this 



end we can still follow the analogy of the photographic camera 

 and process to advantage. What are the conditions requisite to 

 a good photograph? What are the causes of defects in photos? 

 Since photography is now so generally understood it will be 

 permissible to follow this lead in our efforts to explain the 

 general principles of lighting. 



To get a good picture you must first of all have a good lens. 

 By no possibility can the finished photo be any better than the 

 image thrown on the plate: poor plates, or poor developer, or 

 wrong exposure may make it worse. The character of the image 

 is determined solely by the lens. The lens of the eye is fre- 

 quently defective — after middle life, nearly aways so. The cornea 

 often becomes distorted in shape, taking an elliptical instead of a 

 true spherical form, which prevents focussing sharply on hori- 

 zontal and vertical lines at the same time. This is called astig- 

 matism, and is corrected by the use of glasses having a cylindrical 

 surface. After middle age the lenses generally lose their elastic- 

 ity to such an extent that they cannot be focussed on near objects, 

 thus becoming far-sighted. This is corrected by using glasses 

 having a slight magnifying power, i. e., having slightly convex 

 surfaces. Sometimes the lens has too great a curvature, pro- 

 ducing near-sightedness, which is corrected with concave, or 

 reducing, glasses. Not infrequently the two eyes have different 

 focal lengths, or different-shaped corneas, and must be matched 

 optically by the use of different glasses for the two eyes. All 

 defects due to deformation of the lenses of the eye can be cor- 

 rected by the use of glasses; the crystalline lens may even be 

 removed, and its action performed by an exterior glass lens, as 

 is done in cases of cataract. 



PICTURE ON THE RETINA IS WHAT WE ACTUALLY SEE 



As the image on the plate determines the character of the 

 photo, so the image on the retina determines the character of the 

 visual impression. In fact, the retinal image is the only thing 

 that we actually sec; all of our perception of things by means 

 of vision is the result of experience. We know that the objects 

 we see are in their places because we have learned to connect 

 our visual impressions with other sensations, particularly of 

 muscular movement and touch. Anything that changes the image 

 on the retina from its usual form will mislead the mind as to 

 the reality, as in the case of the mirage. A dog, seeing the 

 image of itself in a mirror, believes it sees another dog; a man 

 who had never before seen a mirror would make the same 

 mistake. We can also see things with the eyes shut — the familiar 

 "after images." 



In speaking of the eye as a camera we naturally overestimate 

 its size. In this respect the eye should be compared to the micro- 

 scope camera. The whole apparatus is only an inch in diameter, 

 and the actual size of the image and the extent of the field is 

 shown in Fig. 2. The diminutive circle in the center shows the 

 part of the image falling on the fovea, which is seen sharply. 

 The conscious effort which we make to look sharply at an 

 object is cx|)cnded in turning the eyes so as to bring the images 

 of the object on the foveas — called fixation — and in bringing the 

 lenses to the curvature necessary to produce a sharp or focussed 

 image. 



The impression we have of seeing about us within a wide 

 field is due to the fact that the eye-ball is naturally in constant 

 motion, of which we are as little conscious as of winking, and 

 the different views are blended in the mind by memory. So 

 habitual is this motion of the eye-ball that it can be overcome 

 only by a severe conscious effort, which soon becomes irksome, 

 and then painful, like holding the arm outstretched. Try looking 

 sharply and steadily at a single word on this page, and you will 

 soon appreciate this fact. In this constant roving the eye in- 

 stinctively tends to bring the most prominent, i. e., the brightest 

 point, into focus on the fovea — a fact which will receive attention 

 later in connection with glare. Our little micrographic camera 

 is thus a panoramic camera— the kind that swings on its tripod 



