EXTENSION. While our spatial experi ences undoubtedly contain many elements which are noted for a peculiarly spatial nature, the predominance of psychological opinion is that certain experiences possess the unanalyzable primitive attribute of extension. Though the existence of this attribute is denied by Wundt and is attributed to all sensations by James, it is generally held that it exists and is confined to cutaneous, visual and possibly to kinaesthetic sensations. The "vastness" which James mentions as pertaining to such experiences as those of hearing is in all probability a complex of various degrees of intensity, clearness and other sensory attributes.
While sight and touch are endowed with the attribute of extension, this extension is only two-dimensional. Everything which we see is stretched out over the surface of the ret ina, and everywhere we feel is stretched out over the surface of the skin. The transition from two to three dimensions involves an ap peal to experiences which are neither visual nor cutaneous, and which, in a large measure, are not specifically spatial. In the case of vision these are (1) The difference between the images on the two retina. (2) The experience of strain in the muscles which make the two lines of vision converge. (3) The experience of strain in the muscles of accommodation.
(4) The experience of clearness of vision when the eye is focused on an object, and of blurred ness when it is not. (5) Geometrical perspec tive. (6) Atmospheric perspective. (7) The partial obliteration of more remote objects by nearer ones. (8) The parallax of the various objects of vision, consequent upon motion of the head or of the entire body. (9) The visual angle subtended by known objects. (10) The effect of light and shade.
In the case of touch, the muscle and joint sensations indicating the position of the various parts of the body are of the utmost importance in giving knowledge of the third dimension.
The various tactile and kinmsthetic limina of extension are discussed under LockurY (q.v.). At the fovea of the eye, the lower
linen of visibility, or ((minimum visibile," is probably the angle subtending a single cone or from half to a whole minute of arc. Ob jects subtending smaller angles can be per ceived, and even compared in size, but it ap pears that this is due to the differences in the intensity of the stimulus exert and the sensation they cause, quite apart from the extensity of the latter, and that the real area stimulated is at least an entire cone. The least distance at which two objects can be distinguished is about a minute of arc at the fovea, but near the periphery of the retina it is as great as two degrees or thereabouts. The sensation experi enced when a small point of light is seen often lacks all colors. It takes a larger area of stimulus to evoke color, and often a still larger area to evoke the color which we should nor mally associate with the source of light.
Extension obeys Weber's law (q.v.). The least noticeable increment of a linear visual ob ject is about one-fiftieth the length of the latter, if movement of the eye is allowed. It is con siderably larger when the eye is kept at rest; but as is obvious, measurements with the eye at rest are by no means easy. The field of vision is rather irregular in outline. It extends (from a line connecting the centre of the pupil to the macula lutea) about 80 degrees outward, 65 degrees inward and 65 degrees downward. Of this, about two-thirds can be seen by both eyes. (See SPACE). Consult James W., (Prin ciples of Psychology' (New York 1890); Kiilpe, (Outlines of Psychology' (London 1909) ; Titchener, E. B. (Text-Book of ogy> (New York 1910); Wundt, W., (Grund ziige der physiologischen Psychologie' (Leip zig 1908-11) ; (Introduction to Psychology' (London 1912).