1 Temperature sensations have the attribute of extent (§ 8). But as the temperature sensations which enter into extensive ideas are always combined with pressure, we shall not discuss them separately.
1^4 Perception and Idea in what direction the town hes, what its size is, how its streets are planned, etc. The temporal and spatial attributes of sensa- tion thus become, as it were, detached in the idea from the qualities which they accompany: we can compare the distance from us of a sight and a sound, saying that " that voice comes from the other side of the wall "; we can compare the duration of a taste and a pressure, or the rate of recurrence of tones and flashes. The qualities are here irrelevant: duration and distance are in the foreground.
Intensity, however, has not been able to shake itself free of quality, as duration and extent, ' time ' and ' space,' have done. Intensity is always thought of as the intensity of a particular quality; it would be meaningless to compare the intensities of sunlight and thunder-clap. Mankind has had no need to define intensity, to set up an intensive standard, as it has to define dura- tions and extents. It is enough, in most cases, to know that a light is ' fairly bright '; a taste ' too sweet '; a sound ' exceedingly faint.' Even to-day physics has no satisfactory unit either of light or of sound. Commerce has, it is true, developed a scale of weights, which can be looked upon as varying intensities of press- ure or of the complex of pressure and strain; and we accord- ingly possess the ideas of a ' pound,' a ' kilogramme,' etc. But these ideas are of a very simple nature. They are confined to a single group of sensation qualities, and their names hardly denote more than degrees of se?tsatio?t intensity. They may, therefore, be left out of account here.
I. Extensive Ideas I. Extensive Ideas § 44. Locality or Position. — If we are pressed upon dif- ferent parts of the body, e.g., upon arm and forehead, we are able to indicate very exactly, even when the eyes are closed, the portion of skin affected: we have a clear idea of the locality of cutaneous pressure. As we sit looking at the wall opposite us, we have an equally clear idea of the position of each of the repeated patterns of the paper.
§ 44- Locality or Position 155 And again, if we are suddenly required to shut our eyes and describe the position of our arms, or to state the posi- tion of some part of our body which we cannot see, e.g., of a leg stretched under the table, we find no difficulty in the task: we can form a clear idea of locality or position from sensations of articular pressure.
Method. — (i) Two methods have been employed to test the accuracy of cutaneous localisation, {a) The subject sits, with closed eyes, at a low table. His left arm is laid out, palm upwards, upon the table, and he holds a charcoal pencil in his right hand. The experimenter has a similar pencil, and sets it down for a moment upon the subject's left wrist: the subject, as soon as the pressure is removed, sets his own pencil down upon the same wrist, striking as nearly as possible the spot previously stimulated. Both pencils leave a mark. Hence if the subject has localised inaccurately, we can measure the amount of his mistake, and compare it with the mistakes made by other persons, or by the same individual at other parts of the skin, {b) The object of the second method is to determine how accurately we can localise within one and the same area. The two points of a pair of drawing-compasses are set down together upon the skin. If the distance between them is very small, they are not perceived to be two, but are taken for a single point. The distance separating them must be gradually increased. With a certain separation, they are perceived to be two, i.e., separately locahsed.
When the points are applied in succession (first method), the average error of localisation on the wrist is from 5 to 10 mm. The subject thinks that he has struck the spot previously stim- ulated, when his pencil is in reahty this small distance to one side of the spot. The distance between simultaneously applied com- pass points (second method) which enables us just to perceive their difference, i.e., to locahse them differently, varies for different portions of the skin and for points of different sharpness. The results obtained by the use of exceedingly fine points are: on 156 Perception and Idea the finger-tip,.1 mm.; on the cheek,.5 mm.; on the upper arm, (2) The just noticeable difference of visual position at the centre of the field of vision would be that of objects separated by the minimal visual extent,.005 mm. (§ 24). If the objects are situated in the outlying portions of the field, and their position observed in ' indirect vision,' z>., while the gaze is still directed upon the central portion, our discrimination of their position is far less accurate. To assure yourself of this, use the method described in § 24; but hang the white threads at the right or left end of the grey screen, while you look steadily at a black mark placed at its centre.
(3) The just noticeable difference in the position of a Hmb, the least noticeable difference of ^ articular position,' is smallest in the case of the largest joints. By the shoulder we can perceive a difference of position when the arm has been moved through a dis- tance of.2°; by the wrist no difference of position is perceptible until the hand has moved through.3° (the degrees are degrees of arc described by the moved member with shoulder or wrist as centre). The values for hip and ankle are,.5° and 1° respectively. Special instruments are required for experiments in this department; the member to be moved must be laid out upon a support, and the support must be movable in various directions without any jar and without any alterations in the pressures and strains proceeding from the supported member at the beginning of the experiment.
The physiological conditions of localisation have not as yet been satisfactorily made out. If we transplant a piece of skin from one part of the body to another, the trans- planted piece carries its old locality v^ith it. Thus a piece transplanted from the thigh to the back v^ould still give rise, for some little time, to thigh-impressions. Not until it has thoroughly settled down in its new surroundings does it take on their local character. In the same way, the displacement of a group of nervous end-organs from § 44- Locality or Position 157 one part of the retina to another carries with it a displace- ment of objects in the field of vision, which persists until the displaced organs have taken root again, and acquired a new local value. In some manner, which we do not as yet fully understand, the sense-organ mirrors, in its differ- ent parts, the different positions of external objects.
But we not only localise: we consciously localise, i.e., have an idea of locality. To explain this fact it is necessary to assume that the sensations from skin, retina and articu- lar surface possess each a certain local mark or local sign, — some conscious peculiarity which gives them a definite space value, within the field of touch or vision. Any sen- sation from these three organs has, as a sensation, inten- sity, quality, extent and duration; as a constituent of an extensive idea, it must possess local signature as well. What the local sign is, in any given case, depends upon mental constitution.
Local Signs: (i) Skin. — Not only is the skin, physiologically regarded, a localising organ: the organism is endowed with reflex locaHsing movements. If a spot of skin is irritated, hand or foot moves to it reflexly, in obedience to purely physiological laws. Out of this unconscious localisation the conscious local mark arises, by the following stages, {a) The movement of hand or foot, though reflexly set up, occasions organic sensations in joint, tendon, etc.; so that definite groups of organic sensations become connected with pressures upon particular parts of the body. The local sign may consist, therefore, of remembered organic sensa- tions, {b^ The reflex movement towards the irritated spot will usually be seen; so that the local sign may contain a visual sensa- tion, a picture of the part touched, as weU as organic sensations. (<:) The organic sensations may pass unnoticed, owing to the habitual nature of the movement. The local sign of a pressure will then be a sensation of a quite different order, — a sensation of 158 Perception and Idea sight. (^) Finally, the visual picture itself may disappear, and its place be taken by a word, the name of the part of the body pressed. Often enough, when we say that we remember an occur- rence, we remember only the form of words which describes it. So now, when I am touched upon the arm, there flashes up in my mind the word ' arm,' and this word is the local sign of the pressure.
Method. — Have yourself touched at different parts of the skin. Introspect very carefully, to discover of what processes your own system of local signs is composed. In the first few trials, it may seem to you that the pressure itself has a different quahty in the different cases. But if you look closely, you will come upon the real local sign, probably a visual picture or a word.
Vision is not essential for cutaneous local signature. Those who are born bhnd acquire an idea of the locality of pressures. Their local sign may be {a) a complex of organic sensations; (^) a tactual map or picture of the part touched, plus the organic sensations; {c) the tactual map alone; or (^) a word. The '■ tac- tual picture ' is aroused and perfected by movement of the fingers over the touched spot; its components would be extent of press- ure, i.e., the distance travelled over by the finger before it came to the edge of limb or trunk, certain hardnesses or softnesses of surface, etc. It is not easy for us, who see, to form an idea of such a ' picture '; but it undoubtedly exists.
(2) Joint. — The local sign is here either {a) sl complex of organic and pressure sensations, aroused by the tension of skin and tendons and the contraction of muscle; (/5) a complex of these and visual sensations; {e) visual sensations; or (d) a word.
(3) £ye. — It has been suggested that the original local marks of the retina were also {a) organic sensations. The eyes turn re- flexly towards an object which has suddenly appeared in the field of vision, so that the object is brought opposite to the centres of the retinae, the spots of clearest vision. These reflex movements would give rise to sensations of strain and contraction, and the local mark would accordingly become conscious in the form of remembered organic sensations. There can be little doubt that these sensations are capable of the delicate gradation which would § 44- Locality or Position 159 be necessary if they were to form the basis of the visual idea of locahty. We know, however, (/;) that the same stimulus occasions different sensations, according to the part of the retina upon which it acts. What is red to the centre of the retina becomes bluish as it moves outwards from the centre, and finally, at the extreme edge of the field of vision, passes into black. We do not notice any differences of quality within a field of colour, because we have often moved our eyes over the entire surface of such fields, and thus learned that objective differences do not exist. But it may be, nevertheless, that they constitute the original local signature of the eye.
These ideas of locality are ideas of the position of an impression upon an extended surface. We perceive the place of a pressure upon the surface of the body, the posi- tion of a particular pattern upon the extent of wall before us, the position of a limb within a plane of movement. But we possess other ideas of locality, ideas of the position of an object in three-dimensional space, which include the idea of distance from our own body. We can find where a thing is, in the dark, by stretching out our hand towards it; we can estimate the distance of a visual object from ourselves, or from some other object which we say is be- fore or behind it. The tactual idea of locality, in this second sense, is not hard to explain; the visual idea has been variously accounted for.
The Third DiJucnsion: (i) Tactual Idea. — The tactual idea of distance in the third dimension arises from the connection of extents of cutaneous pressure with the articular sensations called out by movement. The whole body or a bodily member moves towards the object, and comes into contact with it. Hence we have the tactual measures of distance, — foot, span, cubit, etc.
(2) Visual Idea. — The corresponding visual idea has been explained in two ways, {a) The two eyes look at the same i6o Perception and Idea object in space from two slightly different points of view. We can take two photographs of the object from these points of view, placing a camera where each eye would be. Let us paste these photographs side by side upon a strip of cardboard, and lay the strip in a stereoscope, so that the photograph taken by the right hand camera is presented to the right eye and the other to the left. We see one picture only; but this picture is very different from either of the separate photographs. It looks solid: we have an illusion of tridimensionality. From this it has been argued that we perceive distance because the pictures formed upon the two retinae by the same object are different; and that we perceive differences of distance, because the differences between the two pictures increase or decrease, according as the object is near or far. On this view, the perception of tridimensional space follows directly from the bodily conditions of vision; it is a necessary consequence of the double structure and single function of the organ of sight. Because we see one thing with two eyes, we see it as a solid, {b) Another hypothesis lays stress upon the strain sensations which proceed from the tendons by which the eye- muscles are attached to the eyeball. The strain sensations differ in intensity, according as the object upon which the eyes are ' converged,' i.e., to which they are both directed, is situated at a greater or less distance from the body. The nearer the object, the greater the strain of ocular convergence; the more remote the object, the less the strain. In this way, it is said, intensities of strain furnish a measure of the amount of distance.
Method. — To test the discrimination of the eye for distances in depth, we hang a fine black thread midway between the face and a white screen or wall. The thread is gradually moved back- wards or forwards, by an assistant, until a difference of position (distance) is perceived. The subject should close his eyes dur- ing the interval between experiment and experiment, and during the time when the assistant is altering the position of the thread in a given experiment. On opening the eyes, he should look first at the white screen, and from that to the thread: the posi- tion of the eyes and strain of the eye muscles will thus be the same at the beginning of each experiment. The just noticeable § 44- Locality or Position i6i difference of ocular convergence is one-fiftieth of the distance of the thread from the observing eyes ((/. the expression of Weber's law for strain sensations: §§27, 28). It is noteworthy that with a very slight degree of ocular convergence, i.e., when the thread hangs at a considerable distance from the eye, this difference of one-fiftieth corresponds to the least difference of ^ position which the eye can perceive on a plane surface. In concrete terms, if the thread is moved from a distance, say, of 200 cm. to one of 196 cm. (one-fiftieth nearer), the distance separating the two pict- ures which it throws on each retina in its two positions is.005 mm.
This fact seems to show that the sensations aroused by eye move- ments are capable of serving as the conscious local signs of visual sen- sations.
It is impossible, in the present state of our knowledge, to decide between the two hypotheses given above. It may be that both contain a part of the truth, — that eye movement is the primary factor in the idea, but that it is assisted by the difference between the two retinal images. Certainly, the importance of movement for the tactual idea of locality suggests that eye movement may be of similar importance in the sphere of sight. And the number and arrangement of the twelve eye muscles lead us to ascribe some important functions to them, — just as the number and arrangement of the six semicircular canals indicate that they play M M Fig. 6. — The eyes are converged upon the thread a; the thread throws two images upon the two spots of clearest vision, <:, c'. If the eyes are now converged upon the thread at b., the yellow spots will move to the positions </, d'. Under the conditions stated in the text, when the distance a~b is one-fiftieth of the total distance of the thread a from the eyes, the retinal distances c-d 1 62 Perception and Idea some important part in the total adjustment of the organism to its surroundings. The circumstance that in adult life we pay but little attention to the strain sensations aroused within the eye sockets does not count for much: we may have attended to them in childhood, i.e., at a time when we were incapable of introspec- tion; or attention to them may date still farther back, to an earlier stage in the evolution of organic life. Moreover, as our experi- ence grows, we learn to infer the distance of an object by means of certain indirect or secondary criteria (§ 53), so that when the strain sensations had done their work they would naturally be replaced by other conscious processes.
Those who accept the hypothesis of eye movement as correct declare that the apparent solidity of the combined stereoscopic pictures is not due to the bodily conditions of vision. It is not a direct consequence of the fact that we see one thing with two eyes, but rather a matter of habitual interpretation. We see in the stereoscope a surface of broken and irregular outline, and we construct a solid from this surface, by the help of remembered eye movements or of the secondary criteria just now referred to.
Vision is by far the most important of the localising senses. Our idea of the posture or attitude of our body generally takes the form of a mental picture, although it might have been built up from articular sensations; and our idea of the locality of a pressure, or of the position of an object which we ' feel ' in the dark, is as a general rule a visual map of the place touched or of the object among its surroundings. If there is a conflict between the tactual and visual ideas, the visual wins, — we trust our eyes.
Method. — Cross the second finger of the right hand over the forefinger, so that the top joint of the second finger points to the thumb. Take up a marble between the crossed finger-tips. You have two pressures: one from the right-hand side of the second finger, and one from the left-hand side of the forefinger. If the fingers were occupying their normal positions, these sides could § 45- Form and Magnitude 163 not be pressed by the same object; and therefore, if you trust to your tactual idea of locaHty, you must suppose that you are holding not one marble, but two. But so accustomed are we to form a mental picture of what we are touching, that you will not be able at first to get the idea of two objects from the single marble, if you yourself take it up between your fingers. Close your eyes, and let an assistant put the marble in position in the course of a series of experiments with stimuli of whose nature you are not informed. Under these conditions you will judge that there are two objects in contact with your skin; and having thus formed the true tactual idea, will be able to ' feel ' the marble as two even with your eyes open. But you regard it, of course, as one marble: the evidence of sight is believed.
This experiment is as old as Aristotle. It is described in the Aristotelian tract "On Dreams," and the author explains it just as we have done, remarking that " sight stands above touch."
§ 45. Form and Magnitude. — Our ideas of shape and size are, like those of position, of two kinds: superficial, ideas of the shape and size of pressures on the skin or patterns on a seen surface, and tridimensional, ideas of the shape and size of objects in space. Vision can furnish both kinds of ideas. Skin and joint together give us ideas of the form and magnitude of objects of three dimensions. The skin alone cannot do this; if we had no eyes, and were unable to move, our ideas of form and size would be super- ficial only.
A ' form ' is an extent which is bounded or limited in a certain way. When we look at a black mark on a grey surface, the boundary lines of the black mark naturally at- tract our attention: it is there that the contrast between the two qualities begins (§ 38). As the eye follows differ- ent boundary lines, it traverses different distances and rests at points of different position. Different names have been 164 Perception and Idea given to the impressions which call forth in this way dif- ferent complexes of sensation in and about the eye: circle, square, cross, etc. The differences between the stimuli are differences of form.
* Size 'is 'so much ' of a certain form. One square is twice the size of another when the extent comprised within its boundary lines is twice the extent comprised within the quite similar boundary lines of the other figure.
(i) Superficial Ideas. — (<^) The cutaneous idea of form can be tested by applying to the skin surfaces of different shapes (squares, circles, etc., cut from wood or hard rubber). It has been found, e.g.^ that a triangular surface, if applied to the tip of the tongue, must have sides of 3.5 mm. length, if applied to the tip of the middle finger, sides of 7 mm. length, if it is to give rise to the idea of a triangle.
To test the cutaneous estimation of size, apply a series of circles, triangles, etc., of gradually increasing size, to some part of the skin. Two circles are of just noticeably different size for the tip of the tongue if their diameters are.5 and i mm. respectively.
The ' cutaneous size ' of a surface is less than its ' visual size. When, that is, we think of the surface in terms of a passive press- ure upon the skin, we think of it as smaller than it * looks.'
{h) The visual idea of superficial form was originally gained by the help of movement, whether of the eye itself or of the stimulus. Either the eye moved along the boundary lines of the figure, or the figure, contained within its boundary lines, moved across the otherwise unchanged field of vision. After a time, these move- ments became unnecessary. The practised retina is able to dis- tinguish shape at a glance (§ 53).
The just noticeable difference of visual size can be determined by a method similar to that described in § 27, except that, in place of threads, figures cut from cardboard must be used.
(2) Tridimensional Ideas. — (^) The tactual idea of form, an idea derived from the connection of sensations from skin and joint, is capable of a high degree of development. The § 45- Form and Magnitude 165 blind, as is well known, read a '■ raised print ' easily and accu- rately.
{b) The visual idea of tridimensional form is made up of the idea of superficial form////j- the perception of distance.
The ' tactual size ' of an object is generally larger than its 'visual size'; an object 'feels' to the moving hand larger than it looks. The tactual idea itself differs, according to the member by whose aid the estimate is made. The cavity of a hollow tooth seems greater to the tongue than it does to the finger. To both, it is greater than it is to the eye.
The visual idea of the form and size of an object is most prompt and certain when the boundary lines of the object are unbroken; the tactual idea, when they are broken. An object in the field of vision stands up more distinctly from its surroundings if its outline is continuous; but a tactual form stands out most dis- tinctly from its background if the outline is interrupted. Test this by trying to read, with your finger-tips, two sentences, one printed in ordinary raised print, the other in the dotted bhnd- print. It is easier to ' feel ' a raised P when it is printed • * than when it is printed in the form P.
There are two special questions which call for notice under the head of the visual idea of form. These are the questions of the continuity of the field of vision, and of the re-inversion of vision.
(i) The Blind Spot. — When we look out over a landscape, we see it as an unbroken expanse. The field of vision is continuous; there is nowhere any interruption of outline, any gap in the series of impressions. Yet the retina is not sensitive over its whole sur- face. Like the skin (§ 16), it is a mosaic of sensitive points. And the retinal mosaic, unlike the cutaneous, has within it one very large area which is altogether insensitive, — the place of entry of the optic nerve.
Method. — It is easy to assure yourself that you are blind to certain stimuli in the field of vision. Seat yourself at a con- venient distance from a white screen. Close the right eye, and keep the left steadily directed towards a small black disc pasted upon the screen. Let an assistant move a similar black disc, held 1 66 Perception and Idea upon a light rod, slowly across the screen, starting from the point of regard, and travelling towards your left. At first, as you look at the fixed disc, you will see both that and the other: the first in direct and the second in indirect vision. But after a little time, the moving disc will suddenly disappear. Yet it has not passed beyond the Hmits of the field of vision; for if the assist- ant move it still further to your left, there comes a point where it as suddenly reappears. The distance from point of disap- pearance to point of reappearance is the breadth of the blind spot; this can be marked in pencil up- *- on the screen. The form of the blind area can be determined by moving the black disc along all the va- P^iG. 7. — Blind spot of the author's left eye. rious meridians, ver- Reduced from a large diagram, in which the tical and oblique, and distance from the inner edge of the point of marking on the screen fixation, «, to the inner edge of the blind spot n •. r jwas 54.5 cm. Ihe distance ot the pomt of fixa- tion from the eye, in the experiments, was 2\ m. pearance and reap- pearance. Plainly, then, there is here a problem to be solved. The field of vision is broken; yet, in ordinary life, we do not perceive that it is broken. Two explanations have been offered, (a) ' At the blind spot,' it is said, * we do not see afiything. If we saw a hole or gap in the field of vision we should be seeing somet/mtg. As we see nothing, the field must appear to be unbroken.' This explanation might be accepted, were there not experimental obser- vations which tell against it. For instance: we can estimate the distance between two points whose retinal images lie on either side of the blind spot as well as we can that between any other two points seen in indirect vision. Now if the explanation just given were correct, the two edges of the blind spot ought to come together, and two points lying one on either side of the spot to be brought so much nearer each other. Since the blind spot does not interfere with our estimation of visual extent, the space in the § 45- Form and Magnitude 167 field of vision to which it corresponds must be somehow 7f//<f^?//. This can be shown, again, in the following way. If we look at a printed page, under such conditions that the words at its centre fall upon the blind spot, we find that though the central words are not legible, there is visible in their place a hazy whiteness. Something is seen, though the something does not agree with the stimuli actually presented, {b) Evidently, then, the bhnd spot is blind only to peripheral impressions; the area which it occupies in the field of vision is filled up by centrally aroused sensations, of the same general character as those aroused in the peripheral organ — sensations of greyish white, if we are looking at a printed page, of red if we are looking at a red surface, etc. The reason for these central sensations is to be found in the fact that the eyes can move. We have only to sweep our eyes over the printed page to discover that it is an unbroken surface; we can read con- secutively from the top line to the bottom. We have moved our eyes over visual surfaces so often that we cannot help thinking of them as continuous; and this thought is confirmed in every case of actual movement. Here, as in many other cases, we have