EXTENSION. 382 EXTENSION. recording the limit (expressed by the visual angle or the distance between the two retinal images) vhich they are just distinguishable. rhe ision, thus measured, varies with
- 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, llore recent de- terminations by Stratum, excluding the error of dispersion, indicate that a lateral displacement in the relative position of two vertical, end-on-end lines i- 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 Schoute'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 spaee-limen for colors must lie expressed in term- nf three values: (1) the achromatic limen,
- ii 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 la i limen is highest for orange, yellow-green, and violet, lowest for yellow, blue-green, and red- purple: i.e. a relatively huge extent of orange, etc., i- necessary to allow the perception of its true color. For red all three limina coincide. We may regard the pressure spot as the unit, of cutaneous extent, as the retinal cone is that of visual extent. The limen of pressure separation, falsely assumed to be identical with the Liminal extent, was lir-t investigated by E. II. Weber. Weber'- results show clearly the dependence of the limen of separation upon the place stim- ulated. The following values, taken from his ienl table, illustrate this poinl : Tip of tongue. 1 mm.: tip of linger. 2 mm.: cheek, 11 mm.: forehead. 23 mm.: mi. die of back, 68 mm. By the stimulation of individual pressure spots Goldscheider found much lower values: Chin. .3 mm.: cheek, .1 mm.; forehead, .7 mm.; back, 5.0 nun. Subsequent work upon Be thesiometry, or 'Weber's sensory circles,' as thi e experiments are called, ha- emphasized the lav of Vierordl thai the -p.ee limen at any point in the length of a limb i- inver-eh proportional to the di tance of (lie stimulated pari from the axis of rotation, and called attention in the rapid increase of the limen hi separation with fatigue an increase so characteristic as to bi urged by Griesbach as a pracl ieal tesl fur degi I gi neral fal igue h i problem, discriminabilitj . i- tei med in the phere of vision 'eye measurement.' The i the mam in- '■ I I'll inn made are often different depend upon man; fai i lit ion even undei expei imental con ■ ■us. These factors are eye movement the quality of the compared extents (see under Tl.r.nsiox) , their absolute length, their distance from the eye. tlo-ir direction (vertical, horizontals etc.), the use of monocular or binocular vision, etc. Running the eyes along the line-, i.e. the introduction of the strain sensations set up in the eye muscles, appears to aid discrimination of linear extents. When we estimate extent in this way. i.e. partly in terms of intensity, the relative difference limen is constantly at 1/50; two lines seem different if one is one-fiftieth longer or shorter than the ot+ier. Lately Schoute has demonstrated that, with the resting eye. one can discriminate at least four different extents, all of which fall within the limits of a single retinal cone, although the actual basis of this discrimina- tion is the quantity of light which falls upon the cone. The discrimination of tactual, like that of visual, extents, is complicated by extraneous fac- tors; arm-movement measurements involve the factors of the duration and intensity of strain sensations, while in cutaneous experiments proper it is difficult to exclude judgments based upon visual terms. Two circular surfaces applied to the tip of the tongue may be recognized as dif- ferent when their diameters are no more than 0.5 mm. and 1 mm. ; but on the back they must be 2 mm. and 25 mm. respectively. Cold sur- faces appear larger than warm surfaces of equal size. ( >nr provisional assumption posited extent as an ultimate property of certain sensation sys- tem-, as irreducible and unanalyzable as quality or intensity. This view regards the perception of depth arid all other spatial relations as derivative products of associations formed by experience. Thus the quasi-spatial nature of cer- tain sense-qualities, e.g. the seeming differences in the 'bigness' of sounds, does not demand the as- sumption of any elementary spatial attribute in these sensations. It remains to be pointed out that this view of extent has not gained universal acceptance. At the one extreme certain psychol- ogists, notably Wundt. seek to derive all spatial determinations of mental processes from other non-spatial contents of consciousness. (See Fusion.) They do not, therefore, postulate a spatial attribute for any sensation system. At the other extreme certain psychologists, of whom • lames may be considered typical, ascribe an ele- mentary spatial attribute not only to visual and cutaneous, but to all sensations. .lames prefers the term 'voluminousness.' "This element, discern- ible in every sensation, though more developed in - than in others, is the original sensation of space. . . . This 'vastness' is as great in one direction as another. Its dimensions are so vague that in it there is no question as yet ot surface as opposed to depth; 'volume' being the best short name for the sensation in question." From the psychological standpoint tin' merit nf these three positions can be adjudicated only upon the verdict, of trained introspection as aided by the experimental method. The principle of parsimony must incline us In the simplest i ' si-tent theory. From this point of view the merit appears t" lie with those who maintain the mid die position, though from the genetic side 1 1" Wundtian explanation is mosl satisfactory. I" other words, we may consider that the adult human consciousness is unable by introspection to gel behind extent or 'spread out ness' a- an ulti- mate datum of experience, although we may
