MOTIONAL PERSPECTIVE.
To create the sensation of solidity from mere images thrown upon a screen by means of an optical lantern, and without further assistance to the eyes, is seem ingly impossible, in view of the facts relative to binocular perception already pointed out. This, however, is clone by apparatus devised by Theodore Brown, of stereoscopic projection, as generally un derstood, yet it is an arrangement of lighting which permits of binocular inspec tion, requiring, as it does, the full activities of axial and focal accommo dation. In a similar way one person may look into a plane mirror and see another person's reflection, seemingly solid, giving the eyes their full play, as in nature. Such images are seen from a single plane, the surface of the mirror, and, owing to the varied view points of the two eyes, a pair of dissimilar reflections will be received by each observer. An actual relief ob ject, however, is necessary, in the first place, before this class of effect can be seen.
Salisbury, and to understand this it will be necessary to consider briefly sonic hitherto obscure laws of vision with which this system complies. In estimating the distance of a given object in nature, a one-eyed person depends more or less on its magnitude in comparison with other objects, its effect on perspective, its colour, the degree of its definition upon the retina, and atmospheric criteria. If, however, the object is of an animated nature, the speed of its motion adds very important evidence to the judgment. This may be shown by reference to Fig. 852.
Let an observer's eye be situated at L, viewing two soldiers at A and B. While these men remain stationary, the observer will know that A is farther away than 11, owing to its lesser imagery magnitude upon the retina and the other criteria of estimating its distance. If both men march four steps forward, i.e. across the plane of vision, the mind at once gains the further information that the dis placement of the image of B upon the retina has been greater than the dis placement of A, and during the propor tionate movements the mind has thus been able to form an accurate idea of the as the observer makes the various axial ad justments necessary to bring all planes under especial attention. The oscillation of images upon the retina, however, is rendered more or less unnoticeable by the simultaneous focal accommodations that are operative during the process of binocular vision, throwing each plane out relative distance of the two soldiers. From these facts it will be seen that, by the proportionate displacement of ob jects in a composition, i.e. their relative speeds, the sensation of relief may be gained from a series of flat projections upon a screen. Whether the subject is animated or stationary, displacement of objects in proportion to their supposed distance from the observer's eye will bring about the desired sensation of relief when a series of pictures successively so dissimilar are projected upon a screen in rapid succession.