68 THE POPULAR SCIENCE MONTHLY 



flat surface of two dimensions into an infinity reaching in all direc- 

 tions. 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 mofe 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 in- 

 strument 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 sur- 

 face. 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 " cor- 

 responding 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 start- 

 ling stereoscopic effect of such a view. For the first time in his experi- 

 ence the observer becomes enthralled with a perception of depth as a spe- 

 cific factor in objective impressions. It seems to the writer worth re- 

 cording, 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 in- 

 sensibly 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 attri- 

 butes. 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 sub- 

 stratum of a new psychic realm. 



