. The principles of physics. on the retina of the eye ; and its apparent magnitudeis determined by the extent of the retina covered by itsimage. Rays proceeding from opposite extremities of anobject, as AB (Fig. 232), meet and cross each other within A Fig. 232. the eye. Now, as the distance between the points of theblades of a pair of scissors depends upon the angle that thehandles form with each other, so the size of the imageformed on the retina depends upon the size of the angle,called the visual angle, formed by these rays as they enterthe eye. But the size of the visual angle diminishes
. The principles of physics. on the retina of the eye ; and its apparent magnitudeis determined by the extent of the retina covered by itsimage. Rays proceeding from opposite extremities of anobject, as AB (Fig. 232), meet and cross each other within A Fig. 232. the eye. Now, as the distance between the points of theblades of a pair of scissors depends upon the angle that thehandles form with each other, so the size of the imageformed on the retina depends upon the size of the angle,called the visual angle, formed by these rays as they enterthe eye. But the size of the visual angle diminishes approxi-mately as the distance of the object from the eye increases,as shown in the diagram ; at twice the distance the angleis about one-half as great; at three times the distance theangle is one-third as great; and so on. Hence, distance affectsthe apparent size of an object. Our judgment of the size ofobjects is, however, influenced by other things besides thevisual angle which they subtend. MIRRORS. 331 Section REFLECTION OF LIGHT. 291. Mirrors. Images. — Objects having polished surfaceswhich reflect light regularly ( do not scatter the light),and show images of objects presented to them, are calledmirrors. The mirror itself, if clean and smooth, is scarcelyvisible. An image is a picture of an object. According totheir shape mirrors are called plane, concave, convex, spherical,parabolic, etc. Experiment 1. —a. Look at the mirror M through the hole marked Oin the metal band (Fig. 233). You see in the mirror an image of thehole through which you look,hut you do not see the imageof any of the other holes. liaysthat pass through this holestrike the mirror perpendicu-larly and are said to he normalto the mirror. Rays fallingupon an object are called inci-dent rays. The point where a ray strikes is called the point of reflected rays in this case are thrown back in the same lines andthrough the same hole that the incident rays travel. Rays normal to
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectphysics, bookyear1895
