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Page:Popular Science Monthly Volume 21.djvu/53

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These experiments are delightfully surprising when successfully accomplished for the first time. They are well worth the trifling preliminary trouble which they entail. But even this can be in great measure avoided by having a photograph of the picture taken on glass. If you will previously approach the polite photographer in your most charmingly courteous and irresistible style, enable him to perceive the glittering phantom cone reversed in mid-air, invite him to grasp it and give to this "airy nothing a local habitation and a name," and convince him that, if not illusive, it is even more elusive than the merry sunbeam which his camera alone can catch in all its beauty, he will at once be lost in admiration of your magic skill and singular sagacity, and instantly find it impossible to avoid preparing the wonder-working photograph on glass. This he will smilingly present to you in the most enthusiastic and complimentary manner, with evident gratitude for the favor you have bestowed, and the good taste you have exhibited in selecting him as the recipient of your discriminating and exclusive confidence.

The presence of the uncombined images at the sides of the binocular picture, as it stands out in solid relief, is apt to be confusing, because their effect is partially to distract the attention. In Wheatstone's first experiments, he avoided them by looking through tubes, or into a box. In any case, the methods of stereoscopy just described, although by far the most useful in studying the principles of binocular vision, are not usually acquired until after a few trials. When they are once mastered, it becomes easy to discard pencils and other points of fixation, and the voluntary muscular control of the eyes is sufficient for all cases. Wheatstone gave to the world a new revelation in both the science and the art of perspective, when, in 1838, he devised his reflecting stereoscope for the purpose of removing the difficulties involved in stereoscopy by direct vision. Figs. 2 and 3 are exact reproductions of his drawings, representing the front view and ground-plan of his original stereoscope; and, in describing them, we can not do better than again to give his own words: "A A' are two plane mirrors, about four inches square, inserted in frames, and so adjusted that their backs form an angle of 90 with each other; these mirrors are fixed by their common edge against an upright, B, or against the middle line of a vertical board, cut away in such manner as to allow the eyes to be placed before the two mirrors. C C' are two sliding boards, to which are attached the upright boards D D', which may thus be removed to different distances from the mirrors. To facilitate this adjustment I employ a right and a left-handed wooden screw, r l; the two ends of this compound-screw pass through the nuts e e', which are fixed to the lower parts of the upright boards D D', so that, by turning the screw-pin p one way, the two boards will approach, and, by turning it the other way, they will recede from each other; one always preserving the same distance as the other from the