By L. J. R. Hoist.

520. Nature has equipped mankind with two eyes, which enables him to observe and judge the distance between objects, as well as the lateral dimensions of objects. Experience makes it possible to gauge distances accurately, but as soon as we must rely on the vision of one eye only, the faculty of judging distances, if not entirely lost, becomes materially weakened. The explanation is simple. We receive in our brain two different images of the same object, or objects, due to the different relative positions which our eyes occupy with relation to these objects, and their blending in the brain produces the impression of distance between the objects observed. This fact, upon which stereoscopic photography is based, can be proven by a very simple experiment.

521. Open a book before you upon the table and hold one of your fingers, or any other object which will not entirely hide the book from view, about midway between the book and the eyes. Now, first close the left eye and observe which part of the page is hidden from your view; then close the right eye, and observe that when viewing with the left eye a different part is obscured. Repeat this experiment, bringing the finger nearer to the eyes. You will have observed that the distance between the parts of the book that are covered with the finger increases the nearer the finger comes to the eyes, and also that it is quite easy to see with both eyes, not only the finger, but also all of the book, whereas the finger hides the greatest part of the book when looking with one eye only.

522. These apparent inconsistencies are conciliated in the brain, which through life-long training, has learned that they are caused only by the different distances which objects are from our eyes, and then makes us observe their distance. The fact that young children will reach for objects that are entirely out of their reach shows that they have not yet learned to properly combine the vision of both eyes, which moreover explains their frequent unintentional squinting.

523. These remarks will help us to thoroughly understand the principle of stereoscopic photography, which is to first produce pictures such as each eye forms of a view, and then to arrange them for viewing so that they will blend and, by their blending, produce the sense of distance or space. We need the stereoscopic camera to produce the picture and the "stereoscope" to view it.

524. Stereoscopic Cameras

Stereoscopic Cameras. There are a few devices in existence whereby the two stereoscopic images of a view are obtained by means of one lens only, but they offer no practical advantages over the usual two lens cameras, and therefore are only used where the twin lenses are not at hand.

525. There are mainly three different sizes of stereoscopic cameras in use in this country, viz., 3 3/4 x 7 inches, 5x7 inches, and 5x8 inches. Some manufacturers have made smaller sizes, which have met with a great demand from amateur workers; but views made for sale are, without exception, both here and in Europe, 3 1/2 x 6 inches, which dimension must be considered as a standard, and from which we conclude that the 5 x 8-inch cameras are unnecessarily large, and that the 5 x 7-inch cameras are most desirable, owing to the ease with which plates of this size can be obtained everywhere.

526. It should be plainly stated than in stereoscopic photography the actual size of the picture, or of any object in the picture, is of very secondary importance, as the observance of a proper relation between the taking and the viewing lenses makes it possible to produce the impressions of large and even life-size objects, from very-small views.

527. The next point of importance is the most desirable distance at which the lenses should be placed. Some cameras permit of the adjusting of this distance from 2 3/4 to 3 1/2 inches, but in actual practice this adjustment is of very little or no value, and it is most convenient to place the lenses at the standard distance of 3 1/4 inches from center to center. This is recognized to be the standard distance by all manufacturers of stereoscopic between-lens shutters, and is in proper harmony with *the dimensions of the finished picture, which will appear as standing out behind its mount, and produces the effect of looking at the view through an opening or window, thereby materially enhancing the plastic effect. A lesser distance between the lenses tends to destroy this effect, and a greater distance would prevent the proper blending of images in the great majority of stereoscopes of standard manufacture, and also exaggerate the effect of distance beyond the desirable limits.

528. The wide front board necessary to accommodate the two lenses leads many manufacturers to fit their stereoscopic cameras with a front board of equal width as the back of the camera - a very satisfactory design insuring great rigidity. The internal lateral partition is frequently removable, in which instance the camera is equally useful for ordinary single-lens exposures.

529. The necessity of the greatest possible sharpness of definition from the nearest foreground to the furthest distance makes ground-glass focusing highly preferable over reliance on a focusing scale, as only very few operators can "divide" the focus with certainty without seeing the actual image projected by the lens. For this reason the most successful hand-camera stereophotography is done with reflecting cameras. (See Illustration No. 105.)