STEREOSCOPIC VISION A term used in reference to the power to see stereoscopic images coalesced without the aid of any instrument whatsoever. Briefly, it consists in directing the eyes' axes to a remote plane whilst accommodating their focus to a near plane. As this operation involves strain and considerable practice before it can be successfully accomplished, various suggestions have been made to assist the observer to train his eyes in the acquisition of this power. ably the most successful method of acquiring the power to see stereographs stereoscopically without an optical instrument is the following :— Referring to the diagram, cut a strip x in. wide out of both the right- and left-hand prints of a stereogram. The portions lected should show remote as well as near objects, and the foremost object should occupy the centre position in each strip. Obtain a piece of cardboard (preferably dead black) if in. wide and about 6 in. high ; bend this at right angles in the centre, so that half may lie flat on a table, while the other half will stand erect. Place the two strips of view at A B, side by side, supported at the back, and at a distance of about 3 ft. from them place
the cardboard E. Lower the eyes I, R so that they are just above the level of the table top and at a distance from E of about 7 in., and in such a position that the left eye r, sees only the strip A, and the right eye R sees only the strip B, each eye being prevented from seeing the other strip by the shutter card E.
As the two corresponding points in the views A and B have a separation only of 1 in. instead of 21- in. or 3 in. as in ordinary stereoscopic slides, the eyes are easily made to diverge their axes to this slight extent ; and the result is the two pictures combine and stereoscopic relief results. Having succeeded in coalescing A and B in this position, their separation may be gradually increased, until corresponding points reach zt in. This done, ordinary full sized views may be substituted for A B, and when by practice in this manner the observer has learnt the secret of muscular control of the axes of the eyes, he may dispense with the cardboard.