A Manual Of Photography

304 PRACTICE OF PHOTOGRAPHY. ducted with the greatest art and finish to the last perfection, both with regard to its contours, its lights, its shadows, and its colours, can never show a relievo equal to that of natural ob- jects, unless these be viewed at a distance, and with a single eye ; for if an object, as an orange, be viewed by a single eye, all objects in that space behind it, which we may suppose to be included in its shadow, are invisible to that eye; but open the other eye without moving the head, and a portion of these be- comes visible : those only are hid from sight which are included in the space covered by the two shadows formed by two can- dles supposed to be placed in the positions of the eyes. The hidden space is so much the shorter according to the smallness of the object, and its proximity to the eyes. Upon this Mr. Wheatstone remarks :—“ Had Leonardo da A inci taken, instead of a sphere, a less simple figure for the purpose of his illustra- tions—a cube for instance—he would not only have perceived that the object obscured from each eye a different part of the more distant field of view, but the fact would also have been forced upon his attention,—that the object itself presented a different appearance to each eye.” If any of my readers will be at the trouble to look at a simple solid form, keeping the head perfectly steady, with a single eye, and make an outline drawing of the image as seen— say, first with the left eye, and then with the right eye it will be found that two dissimilar forms will be obtained analogous to those represented in the following diagram. 83. By a little effort, it is easy, by squinting, to resolve these two figures into one, when it will be found that an apparently solid image is formed from these dissimilar outlines of a solid. The stereoscope of Professor Wheatstone is arranged to pro- duce ibis in a more effective manner. The instrument, fig. 84, consists of LAo plane mirrors, so adjusted that their backs form