Identifier: anatomyphysiolo00hewe (find matches)
Title: Anatomy, physiology and hygiene for high schools
Year: 1900 (1900s)
Authors: Hewes, Henry Fox, 1867-
Subjects: Physiology Health education (Secondary) Human anatomy
Publisher: New York : American Book Company
Contributing Library: The Library of Congress
Digitizing Sponsor: The Library of Congress

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st people cannot see objectsclearly which lie nearer than five inches from the eye.This is because the lens cannot be made convex enough tofocus all the rays from such an object upon the retina. Therays from the sides of the object go by. Some people, how-ever, see close objects better than more distant ones. These 250 PHYSIOLOGY AND HYGIENE are shortsighted people. Their peculiarity is due to thefact that the retiua is farther off thau normal from thelens, so that near objects can be bent to it. Owing to thisdistance of retina from lens, these people cannot focus adistant object. No matter how much the lens is flattenedit focuses ill front of the retina, where the normal retinawould be. To correct this difficulty such people have glassesmade with a concave lens. This throws the rays outwardand causes them to focus farther back. In a long-sighted person the ball is shorter than the rule,and near objects focus behind the retina. An additionalconvex lens has to be used to correct tliis.
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Diagram of refraction,o, object; b, lens; c, image upon retina. Action of the iris. The lens is not a perfect instrument.Some of the rays are brought to a focus in front of most ofthe rays. To remedy this defect the eye is providedwith the iris curtain. This curtain shuts off more or lessof these rays which do not focus on the retina, and thusleaves a clear image there. The amount of light whichmust be cut off differs with the distance of the image. Ifthe image is near, much light is cut off; if distant, little THE SPECIAL SENSES 251 light. To meet these varying conditions the iris is made togrow larger or smaller, so that when an object is far awaythe iris is pulled back and the pupil is large, and when theobject is near the iris closes and the pupil is small. You cansee the pupil change if you watch the eye of some one whois looking at your finger as you draw it gradually away fromhis face. To change in this way, the iris is provided with asphincter (Greek spMggeiu, to contract) mus

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