. A treatise on anatomy, physiology and hygiene : designed for colleges, academies and families. light mainly effected ? 930. When we viewobjects at a distance, what kind of lens is required ? Has the eye thepower of adapting itself to different distances ? How is it effected ? A PHYSIOLOGY OF THE ORGANS OF VISION. 409 Gljservatinn. It is well known that a separate imaf^e isformed on cicli eye, and, if,they are not in the same direc-tion, the objects will appear double. This is proved bypressing one eye, so that tlie rays of light cannot enter it inthe same direction as they do in the other; c
. A treatise on anatomy, physiology and hygiene : designed for colleges, academies and families. light mainly effected ? 930. When we viewobjects at a distance, what kind of lens is required ? Has the eye thepower of adapting itself to different distances ? How is it effected ? A PHYSIOLOGY OF THE ORGANS OF VISION. 409 Gljservatinn. It is well known that a separate imaf^e isformed on cicli eye, and, if,they are not in the same direc-tion, the objects will appear double. This is proved bypressing one eye, so that tlie rays of light cannot enter it inthe same direction as they do in the other; consequently, thevision is double. 931. By the action of the muscles of the eye, it is turnedii] different directions, so that objects can be examined uponeach side, as well as in front, witliout turning the body. Bythe slight or intense action of the straight muscles, the eye ismore or less compressed, and the form of the globe ischanged, together with the relative positions of the differenthumors. This modification also adapts the eye to vievv ob-jects at different distances. Fix. 143. ?. Fij. 143. ], A pen, an inverted image of which is painted on the retina of thoeye, at 2. The image of all ohjects upon the expansion of the optic nerve, is invertedby llie •rossirsg of the rays of ligiit from ohjects as they traverse the pupil. Ohservation. If the eye is fixed for a time on some objectwhich is distinguished with difficulty, there is a painful sensa«tion, similar to that experienced by other muscles of the bodywhen used too long. This is called straining the eye. 932. When the refraction of the rays of light is too great,as in over-convexity of the cornea, or the crystalline lens, ortho vitreous humor, or all of them, the image is formed a Wh^t does fig. 143 roiirescnt ? 931. Why can we see objects at the sideas well as in front of the eye, without turning the body ? What is thseffect when the eye is fixed on an object that is indistinctly seen ?35 410 ANATOIMY, rilVSlOLOGV
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