THE PHENOMENON OF STEREOSCOPIC VISION.
Stereoscopic vision, by which is meant the uniting of dissimilar pictures with the two eyes without any optical agency, appears to be possessed by few people, but, by a little practice, the majority may acquire the power. By crossing the axes of the eyes at a point closer than that distance at which the picture is situated, superposition of the two dis similar images may be accomplished. This is precisely the principle and effect of the lensless stereoscope invented by Mr. Elliott (see Fig. 840). The stereoscopic effect obtained by crossing the axes at a near point, however, makes it necessary that the prints, to be so viewed, should be transposed as regards right and left, so that pictures mounted in the usual manner for the stereoscope are not suit able for examination. The better and more convenient kind of stereoscopic vision is that which enables the stereo gram to be examined with the pictures mounted in the usual manner, i.e. the pic ture seen in binocular vision by the right height as the original view. The portion selected should contain distant as well as near objects, a foreground object should occupy the centre position in each strip. Now provide a piece of cardboard (preferably black) It in. wide and about 3 in. in height. Reference being made to the diagram Fig. 846, place the two strips of view A and B side by side on a table in a vertical position, resting against a box or any other support D. At a distance of about 3 ft. from these strips of view place the piece of black cardboard E also in a vertical position. This may be done in the manner shown in Fig. The eyes of the observer are then placed at L a, at a distance from the card a of about 7 in., and in such a position that the left eye L sees only strip A, and eye placed on the right-hand end of the mount. This necessitates that the eyes
must be directed to a remote plane while their foci are accommodated to a near one. Although this is rather an un natural adjustment of the eyes, it is in no way harmful, and may be accom plished in the following manner.
How TO ATTAIN STEREOSCOPIC VISION.
Cut a strip, 1 in. in width, out of both the left- and right-hand prints of a spare st ereogram, leaving them the same the right eye n sees only strip B, each eye being prevented from seeing the other strip by the intercepting card E. As the two corresponding points in the pictures A and B have a separation of only 1 in., instead of 2:1 in., as in the ordinary uncut stereogram, the eyes are easily made to unite these two points, thus superimposing the two strips of view A and B. This will, of course, result in a composite of the two, and stereoscopic vision will have been acquired. Having succeeded in uniting the strips of view in this position, their separation may be gradually increased by shifting them farther apart, until the dis tance between corresponding points is 21 in. This clone, the strips may be ex changed for full-sized pictures, and the experiment repeated with the aid of the card E till the observer has acquired sufficient control over the muscles to enable him 10 hold the eyes in the position required without its use. If at any time afterwards it is desired to view stereograms without the aid of an inslru went, and any difficulty is experienced, the first and second fingers of the right hand may he used in place of the card E.