EXTENSION, EXTENT (Let. exica.sio, from extendere, to stretch out, from cx, out + tenderc, to stretch). The simplest spatial deter mination of mental processes, as duration (q.v.), is their simplest temporal determination. Not all psychologists are agreed upon the existence of extent as an intrinsic attribute of sensation; but for the present purpose we may provisionally as sume that extent, in the sense of 'spread-out ness,' is a characteristic feature, at least, of vis ual and cutaneous and articular sensations. The validity of this assumption will be discussed later. Of tire psychological problems concerned with spatial relations, the majority. such as the perception of form, of distance, position. order, arrangement, the geometrieal-optical illusions, etc., take us beyond the treatment of extent as an attribute of sensation. Within the sphere of sensation there are theoretically three problems to be solved: (1) that of the smallest noticeable extent; (2) that of the largest noticeable ex tent; and (3) that of the just noticeable differ ence of extents, or the sensible discrimination (q.v.) for extent. These problems must he at tacked in the sphere both of visual and of hapti cal sensations, though the latter are, for practical reasons, limited to a single quality, pressure.
The second question is at once the least im portant theoretically and the least difficult of solution. !Maximal extent of pressure may be obtained, e.g. by immersion of the entire body in water. The 'maximum visible' is obviously meas ured by the total area of the 'field of vision.' This embraces for a single eye a region delimited by the following angles (based upon the straight line joining the centre of the pupil with the centre of the 'yellow spot') : outward. 70°-85°; inward, 60°-50°; upward. ; downward, 65°. Hence the maximal field is, for a single eye, an oval measured by a visual angle of 130° 135° horizontally, and 110°-120° vertically.
The first problem, that of the smallest notice• able extent, brings us to the well-worn question of the 'minimum visible.' Two factors must be constantly regarded: (1) In the neighborhood of the stimulus-limen, extent and intensity play into each other's hands. An imperceptibly small area may become perceptible with increased illu mination; an imperceptible degree of illumina tion may become perceptible with an increase in its area. (2) Owing to the error of dispersion
(diffusion of stimulation upon the retina), the extent of the object used as a stimulus may not correctly indicate the extent of the stimnlated portion of the eye. In the method of Helmholtz the least noticeable visual extent is determined by approximating two luminous points or lines placed at a constant distance from the eye, and recording the limit (expressed by the visual angle or the distance between the two retinal images) at which they are just distinguishable. The cnot ss of thus measured. varies with the part of the retina stimulated, being one minute of arc (or 0.004 mm. between images) at the fovea, and decreasing to 1/100 of this value at 30'-40° from the fovea. 1\lore recent de terrainntions by Stratton, excluding the error of dispersion, indicate that a lateral displacement in the relative position of two vertical, end-on-end lines is detected when the visual angle is but seven seconds of arc. Hence, it is argued. the minimum visible' is distinctly smaller (0.0005 mm.) than the diameter of a single cone (esti mated at from 0.0015 to 0.0044 mm.). But it is doubtful whether these figures, expressing as they do the limen for separation of points, give us any indication of the least visible extent; any more than the least distance for the cutaneous discrimination of two points can be regarded as the least perceptible cutaneous extent. The judg ment of two lines or two points does not neces sarily carry with it any reference to space. We shall do better, perhaps, to consider the retinal cone as affording the unit of visual extent; al though, as S•houte's experiments indicate (see below). a limited extent may be less, under certain conditions, than the diameter of a single cone. The limen is further dependent upon the quality of the stimulus. Lane has shown that the space-limen for colors must he expressed in terms of three values: (1) the achromatic limen, at which brightness appears; (2) the chromatic limen. at which color of any sort is seen; and (3) the 'characteristic' limen, at which the actual color-tone of the stimulus is perceptible. The last limen is highest for orange, yellow-green, and violet, lowest for yellow, blue-green, and red purple ; i.e. a relatively large extent of orange, etc.• is necessary to allow the perception of its true color. For red all three limina coincide.