DISTANCE, or DEPTH, l'EurrrtioN \\ hen we look at objects We think that we how far away they are. 'let their distance is not directly given with the act of vision. It is true that a few writers. e.g. Hering and James, be lieve that the pliolitnr field of view was not alto gether tlat. and that distance is 'a genuinely opti cal but the majority agree that it is de rivative, built up by the ;Association of tactual ideas with certain 'signs.' visual and strain sen sations arising in the use of the eyes. lit either ease it is admitted that the 'optical signs' them selves, the of as they are called, are essential to the complete development of the idea. We must distinguish between the 'primary; immediately given. and the dary.' or indirect criteria. The former are j 1 dissimilarity of retinal images; (•1 converg ence strains: (3) ;Accommodation strains; and CO dispersion circles. The last must be taken together with the third. .\lone. it is of minor importance; 1.1r aceommodation, which charities moderately distant object•. will for both nearer :11141 farther objects The strain set up in the ciliary musele as it focuses the lens may. however. serve as a definite empirical criterion of distance. The second fac tor results from the functioning of the eye, ill bilnuallar vision. we are to have a single clear image of near objects, the eyes must eonverge.
intensity of the strain sensations thus •n gendered increases in proportion to the proximity of the fixated point. Experimental investiga tions of the sensitivity to strain sensations. as well as the anatomieal arrangement of the twelve eye-muscles, warrant the assumption that such sensations may give ;Accurate indications of the distance of objects. at least up to a few hundred l'onvergenee. together With t ion. is, according to NV1111(11 and .\ rrer, the es• sclitial basis of the perception of depth. 11111
other authorities. notably VI'lleatstone. lIcring, and II illebrand. lay stress upon the dissimilarity of ret inal images. N•e have given 111 11111* two pictures of every object seen. The differ. between the two 111011re. inereases with tile nearness of the object. The of I he part played by this disparity in the perception of distance led to the 111Vellt 1011 of the stereoscope, which enables us artificiallv to reconstruct the actual retinal conditions of binocular vision.
In our daily life, it is perhaps true that the secondary criteria become most important; they are at least most in evidence. These indirect data of distance are seven in number: 1 I ) lin ear perspective, i.e. the perspective of drawing. the course of the contour lines of objects in the visual field: (2) aerial perspective, i.e. distinct ness of outline and color-tone; (3) the number of intervening objects, or the partial covering of the distant by the nearer: (4) movement of ob jects in the visual field, e.g. swiftly moving ob jects seen from the window of a moving train are known to be near; (5) movement of our own bead or body, with consequent. relative dis placement of objects differently distant: III) dis tribution of light and shade, e.g. the illusion of depth given by appropriately shaded stage-set thugs: and (5) visual angle, i.e. the apparent size of known objects. The two last are the most influential. Consult: Berkeley, An ESMI !I To ward a New Theory of Vision (London, 1709) ; Wundt, Human and Animal Psychology, trans. by Creighton and Titchener (London. 189ti) ; Titehener. An Outline of Psychology (New York, 1S99) ; James. Principles of Psychology (New York, 1890).