The essentials of healthA text-book of anatomy, physiology, hygiene, alcohol, and narcotics . l still be formed, but no sensation of ini: sKNsi; OF SIGHT. 301 light will reach the brain. Therefore, as it is the photog-rapher who sees and not his camera, so it is the brainwhich sees and not the eye, for the eye is simply the camera. Functions of Parts. The eyeballs may be likened to aroom, with a single window in front. Just back of thewindow hangs a dark curtain with a round opening init. All light entering the room must come through thewindow, pass through the opening in the curtain, andstrik
The essentials of healthA text-book of anatomy, physiology, hygiene, alcohol, and narcotics . l still be formed, but no sensation of ini: sKNsi; OF SIGHT. 301 light will reach the brain. Therefore, as it is the photog-rapher who sees and not his camera, so it is the brainwhich sees and not the eye, for the eye is simply the camera. Functions of Parts. The eyeballs may be likened to aroom, with a single window in front. Just back of thewindow hangs a dark curtain with a round opening init. All light entering the room must come through thewindow, pass through the opening in the curtain, andstrike the opposite wall. The transparent cornea is thewindow; the iris is the dark curtain for regulating theamount of light; and the pupil is the central choroid is black to prevent the reflection of lightwithin the eye, and to absorb any light which may passthrough the retina. The sclerotic is hard and firm,for a protection to the eye and for the attachment ofmuscles. The Blind Spot. All parts of the retina are not equallysensitive to light. One spot on it, where the optic nerve. Fig. 104. A diagram for illustrating the existence ot the blind spot. enters (see Fig. 103, B), is entirely insensible to is called the blind spot: it does not interfere withvision because it is impossible for the light from an objectto fall on the blind spot of both eyes at once. If certainrays fall on the blind spot of one eye, they will fall on 302 THE ESSENTIALS OF HEALTH. a different part of the retina of the other eye. But ifone eye be closed there is always some portion of the ob-ject before us which is invisible. This is easily provedby looking at a sharply defined object after the followingmanner: Close the left eye and look steadily at thesmall white circle to the left of Fig. 104: it is possiblenow to see the large white circle even when the eye isfixed on the smaller one. Hold the book vertically ona level writh the eyes at a convenient distance. Nowmove the book slowly
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectphysiology, bookyear1
