flat surface of two dimensions into an infinity reaching in all directions. If the unlikeness of the retinal images as seen simultaneously with the two eyes is really the physical basis of our visual perception of the third dimension of space it would naturally be suspected that the depth perception must become more lively the greater the unlikeness of the two images, up to a certain point. Manifestly the binocular images must depart from similarity in proportion as the eyes are further apart.
Pursuing this idea, Helmholtz contrived a most ingenious instrument the "telestereoscope," by which the distance between the eyes of an observer can be virtually increased to any extent. In its simplest original form the telestereoscope may be reproduced by joining at right angles the edges of two small squares of silvered glass which are then set into the middle of a strip of board having a length of, say, three feet. When the eyes are brought close to this rectangular mirror so that its edge is parallel with the bridge of the nose, it is evident that the right eye sees only the reflection of objects to the right of the field of view and the left eye those in the corresponding area on the left. At each free extremity of the board another, larger mirror is placed, so fastened by hinges that one mirror shall be movable round a vertical and the other round a horizontal axis. These terminal mirrors have their reflecting surfaces turned outwards, away from the observer.
In using the instrument the experimenter brings his eyes close to the fixed rectangular mirror so that they look into either reflecting surface. Now the terminal mirrors are focused on some distant object, as a tree, and it is easy to bring the reflections of the two images on "corresponding points" of the retinas and the distant object appears single but as if viewed by a pair of eyes separated by a distance of three feet. No one can realize, without having experienced its influence, the startling stereoscopic effect of such a view. For the first time in his experience the observer becomes enthralled with a perception of depth as a specific factor in objective impressions. It seems to the writer worth recording, as a suggestion in esthetic pedagogics, that after continued experimentation with this apparatus for some weeks, during which all manner of solid objects occupying the landscape was studied, there insensibly grew up in him an esthetic appreciation of depth, per se, which gave to all solid objects, viewed with the unaided eyes, a charm which immensely enhanced the pleasing combination of their natural attributes. The beauty in nature called more insistently from all her creatures. To sum up, in brief, the very dissimilarity of the retinal images which would seem to subvert the acuity of binocular vision is not only without disadvantage thereto but forms the physical substratum of a new psychic realm.
