Human physiology : designed for colleges and the higher classes in schools and for general reading . y Professor Wheatstone, THE of the stereoscope. 303 which he calls the stereoscope. In using this instrument, youlook at two pictures of the same object with the two eyes, andyet you see but one thing—that is, but one impression is pro-duced in the mind, although two different pictures are made inthe two eyes, and of course two different impressions are con-veyed to the brain. Suppose the object represented is a book,as described in the experiment alluded to, in § 454. In therig
Human physiology : designed for colleges and the higher classes in schools and for general reading . y Professor Wheatstone, THE of the stereoscope. 303 which he calls the stereoscope. In using this instrument, youlook at two pictures of the same object with the two eyes, andyet you see but one thing—that is, but one impression is pro-duced in the mind, although two different pictures are made inthe two eyes, and of course two different impressions are con-veyed to the brain. Suppose the object represented is a book,as described in the experiment alluded to, in § 454. In theright half of the instrument is a representation of the book, asseen by the right eye, and in the left half is a representation ofit as seen by the left eye. As you look at them you see butone book, just as you do in holding the book before your two different images formed in the two eyes are the samein the two experiments. The same thing is done with otherobjects. Thus, the two representations of a dog, seen in 2, are seen in the instrument as a single dog. You observe FIG. that they are shaded differently. They are representations ofthe two pictures, which a dog in this position would make onthe retina in both of the eyes of a person looking at you look at them in the instrument, the single dog thatyou see stands out more than either of the two representations,as seen when they are not in the instrument. The reason isobvious. In the two images formed in the eyes, as you lookinto the instrument, are all the lines of light and shade, which 304 HUMAN PHYSIOLOGY. Rude imitation of the stereoscope. you would see in looking at a real dog with both eyes; whileeither one of these representations contains only a part of theselines. You can imitate in some good degree the effect of thestereoscope, by placing the end of a small book between thesefigures, and letting the other end rest against the nose and fore-head, thus separating the eyes from each other
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