. Human physiology : designed for colleges and the higher classes in schools, and for general reading. a continued com-parison of experiences that he learns the sizes, shapes, and dis-tances of objects. And in doing this, the sense of touch actsas the educator of the vision, very much, as the ear educates thevoice. And even the adult, with all the training which he hasbestowed upon his eyes, often makes mistakes, especially in rela-tion to magnitude and distance. There are various degrees ofskill in seeing; and he is the most skillful seer vrao makes thefewest of the mistakes referred to. 459.
. Human physiology : designed for colleges and the higher classes in schools, and for general reading. a continued com-parison of experiences that he learns the sizes, shapes, and dis-tances of objects. And in doing this, the sense of touch actsas the educator of the vision, very much, as the ear educates thevoice. And even the adult, with all the training which he hasbestowed upon his eyes, often makes mistakes, especially in rela-tion to magnitude and distance. There are various degrees ofskill in seeing; and he is the most skillful seer vrao makes thefewest of the mistakes referred to. 459. Let us look now at the means by which we gain theexperience that is necessary to correct vision. One means isthe appreciation of the space occupied by objects in the fieldof vision. This is measured by what is termed the visualangle—that is, the angle which is formed by two lines comingfrom the extremities of an object, and meeting in the eye, asrepresented in Fig. 174. In this wav we get the idea of mag- THE EYE. 307 Visual angle. Distance of objects estimated by their distinctness. FIG. nitude. But it is manifest that it cannot alone give us this ideicorrectly. It would do so, if all objects were at an equaldistance from the eye. But you can see by the figure, that ifthey are at different distances, you must know something ofthose distances, to estimate the magnitude of the objects bythe visual angle, which they subtend. The arrow at A, B willappear just as large as the larger one at A, B, because it willoccupy the same space a, b on the retina, and subtend the sameangle. But if you know that the one is nearer to you thanthe other, you make allowance for this in the estimation of thesize. Your hand, held up to keep the rays of the sun fromyour eyes, would look to you as large as the sun itself, if youdid not know how near it is to you; and the sun and moonappear to us to have about the same magnitude, because we donot keep in mind the fact that the sun is ninety-six
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