COLOUR-SENSATIONS.

119

colour-sensations that these cannot be equally elementary or fundamental. Thus white which popularly, and even by some savans as Goethe, is regarded as a distinct colour is really composite, at least so far as the underlying nervous process is concerned. Not only so, we may account for all the known variety of tint by assuming three or four fundamental coloursensations. The colours which subjectively appear most distinct and elementary are red, yellow, green, and blue.1

(d) Other Peculiarities of Colour-Sensations. Among the many facts brought to light by modern physiological research in the domain of colour-impression, one or two of special psychological significance demand a bare mention. Of these the first is the relation of intensity to quality. A hue may be modified not only by an admixture of white light but even by a mere increase in the intensity of light. Thus at the maximum intensity all varieties of light tend to have a whitish appearance, at the minimum a blackish hue, which change is evidently a qualitative one. In some cases change of intensity gives rise to a modification of tint which is marked off by a separate name. Thus brown' is merely the effect produced by yellow or red light of a weak intensity.2

Again, colour-sensations are known to be modified by the previous activity of the nerve-elements engaged, as well as by the simultaneous activity of other and adjacent elements. These effects are dealt with under the head of colour-contrast, successive and simultaneous. After the stimulation of the retina the sensation does not instantly cease. It may continue in the form of a positive after-image,' as when we retain an impression of a very bright object, an effect which illustrates the general fact that the nervous excitation set up may outlive the process of external stimulation. This positive image may then give place to a negative one where instead of the original

1 Such elementary colour-sensations must not be confounded with the 'primary colours' recognised by workers with pigments.

2 Qualitative changes also occur when the duration of the stimulation is varied. Thus when the intensity is low more time is needed to produce a sensation of saturated colour than when it is high. The range of nervous elements acted upon appears also to affect the quality of the sensation. (See Ladd, op. cit., p. 334.) The fact that colour-quality depends in part on intensity has led Wundt to represent the colour continuum as one of three dimensions, viz., as a sphere (op. cit., i. p. 471).

colour we have the complementary hue. This last effect, "successive contrast," illustrates the action of nerve-fatigue. The process of stimulation has paralysed for a moment the function of the retina answering to the positive colour-sensation, so that it is only capable of functioning in the opposite way. Simultaneous contrast occurs where one colour modifies a contiguous hue by rendering it more unlike, or sending it further away in the colour-scale. The exact physiological significance of this is not understood. Finally, reference must be made to the great variations in colour-discrimination that occur among different persons. Some people are colour-blind, that is, incapable of distinguishing colours. Such colourblindness is known to exist in various degrees.

27a. Elementary Colour-Sensations. The attempt to derive the multiplicity of colour-sensation from a few fundamental impressions, and at the same time to explain the phenomena of colour-contrast, colour-blindness, etc., has given rise to various physiological hypotheses respecting the structure and mode of activity of the retina. Among these the most popular is known as the Young-Helmholtz theory. According to this, the nervous elements of the retina consist of three kinds of fibre. These are acted upon more especially by the red, the green, and the blue or violet rays respectively. These three colours would thus be in a peculiar sense elementary colour-impressions, while other colours, as purple, bluish green, together with white, would be composite. These three classes of nervous element must be supposed to be unequally distributed over the retina, being only fully represented in the central region, and also to be incomplete in the case of the colour-blind. According to a second theory, that of E. Hering, there are two kinds of nerve-element. These structures, again, are capable of two antagonistic modes of activity, to each of which a distinct colour-impression corresponds. Thus we have four simple or leading colour-sensations. One kind of element is concerned in the sensations blue and yellow, and another in the sensations red and green. In addition to these two varieties of nerve-element Hering postulates a third, the two opposed processes in which underlie sensations of white and black. This hypothesis aims at obviating some of the difficulties of the Young-Helmholtz theory. It is recommended by the circumstance referred to above, that it erects into elementary or fundamental colour-impressions four varieties which we are all accustomed to regard as leading and distinct colours. In its turn, however, it gives rise to special difficulties.1

ance.

These hypotheses have rather a physiological than a psychological importThe fact of a colour-sensation being shown to be the psychical product of the stimulation of a number of nervous elements is psychologically unimportant so long as the colour-sensation itself remains indivisible. This is certainly true of

1 Another hypothesis, not easy to explain in a few words, is given by Wundt The several rival hypotheses are carefully compared by Ladd,

op. cit., i. p. 493 f. op. cit., p. 338 ff.

PHYSIOLOGY OF COLOUR-SENSATION.

121

particular results of such nervous compounding, e.g., the sensation white, for in this case no amount of separate experience of "constituent sensation," e.g., blue, yellow, enables us to recognise them as present in the "compound". The physiological hypothesis only becomes psychologically significant when it helps us to account for observable differences in the sensations, an advantage which, as just remarked, appears to distinguish Hering's hypothesis.

§ 28. Extensity and Plurality of Impression. In addition to these numerous differences of intensity and quality, the sensations of sight are characterised by a fine discrimination of points and extensive magnitude. And it is this circumstance, together with another to be spoken of presently, which gives sight so distinct a superiority to hearing as an intellectual or knowledge-giving sense. The retina is an extended surface, on any point of which (owing to the peculiar structure of the eye) an isolated optical effect may be produced. The sensations received by way of different points of the retina are, as already pointed out, supposed to be from the first distinct one from another in 'local' character or colouring, and it is by means of this separateness that we are able to estimate so nicely the extensive magnitude of a visual impression. The fineness of this discrimination is, like that of colour-discrimination, greatest in the central region, the area of perfect vision. With a view to measure this aspect of retinal sensibility in this region, experiments were carried out by means of two lines placed at a certain distance from the eye and brought gradually nearer one another. These show that in the case of a practised eye two points are distinguished when the visual angle is from sixty to ninety seconds, that is to say, when the retinal images are from 004 to 006 millimetres apart. In the side portions of the retina this fine local discrimination rapidly falls off.

This may be seen in the following table, in which the results of looking at two squares one metre from the eye are recorded:

This decline in discriminative ability does not progress with perfect regularity, and is not equally rapid in all directions. An attempt has been made to connect these limits of discrimination with the magnitude of the terminal appendages of the

optic fibres, viz., the rods and cones. Since the cones are densely packed in the area of perfect vision while they become less numerous and give way to rods towards the periphery, it seems probable that the former are the structures specially concerned in the discrimination of points. Measurement of these cones goes to show that their diameter corresponds (roughly) to the limits of this discrimination.1

MOVEMENT AND MUSCULAR SENSE.

§ 29. Demarcation of Muscular Sense. Sensations are supplied us not only by way of the familiar sense-organs when stimulated by external forces, but also by our own muscular actions. Such actions are important elements in conation, and as such will have to be spoken of presently. Here we are concerned with them merely as contributing presentative elements, analogous to those of tone, colour, etc., which enter into our intellective processes.

Muscular sensations may be defined as those characteristic modes of consciousness which are specially connected with the stimulation and the contraction of the voluntary muscles, as those of the limbs, the eyes, the vocal organ. If, for example, I flex my arm or turn my eyes to the right, or exert my vocal and respiratory organ in the act of shouting, I have a peculiar sensational consciousness by means of which, independently of any mediately resulting changes of tactile, visual, or auditory sensation, I know that I am making a muscular exertion or am actively energising and also something respecting the special character of this exertion. Muscular sensations are thus, though closely conjoined with sensations of the special senses, more particularly those of touch and of sight, sensations sui generis. They are marked off from other sensations as active from passive states. Sensations of light, sound, and so forth precede and call forth muscular action: muscular sensations, on the other hand, are the concomitant or result of such action.

These sensations, though in the adult consciousness apparently simple, are in reality highly complex. They probably consist in part of the psychical concomitants of the process of innervation or motor stimulation itself, viz., "sensations of innervation," which presumably contribute something of its

1 For a fuller account of the experiments respecting the discrimination of points by the retina, see Wundt, Physiol. Psychologie, ii. cap. xiii. § 1, p. 86, etc.

THE MUSCULAR SENSE.

123

characteristic quality to our muscular experience, viz., conscious activity or active consciousness, a factor to be considered fully later on.

At the same time, it is now certainly known that these sensations of innervation, even if they are a co-operant factor at all, are by no means the only one involved. The large part of our conscious muscular experience, as when we move a limb, is made up of the sensational results of afferent nervous processes. That is to say, our muscular, like our other sensations, are, in all normal cases, partly the product of a stimulation of peripheral organs. It is known that whenever we use our muscles a number of sensitive peripheral organs are engaged. Among these may be named the tendons, the joints, the skin which is stretched or folded during movement, and possibly also the muscles themselves into which certain physiologists think they have traced sensory nerve-fibres. According to this view, muscular experience is a complex, made up of the psychical correlatives of efferent and afferent nerve-processes.1

§ 29a. Theory of Muscular Sense. The precise nature and physiological conditions of muscular sensations are as yet only very imperfectly understood. The sensations being so closely connected with those brought about by stimulation of the nerve-fibres of the skin, it has been held by some that there is no distinct muscular sense at all, but that the so-called muscular sensations are really skin-sensations. Opposed to this view is the theory that muscular sensations are sharply marked off from passive (afferent) sensations by being the concomitants of the process of central motor innervation, and so efferent. This view, advocated by Dr. Bain and others as supplying a physiological ground for the fundamental distinction between active experience (consciousness of exertion, strain, etc.) and passive experience, has been affected by recent researches, pathological and experimental. The former consist of inquiries into the power of patients suffering from loss of skin sensibility or anæsthesia to carry out movements of their limbs, distinguish weights, etc., and also into the survivals of the muscular consciousness under the form of a sense of effort, and even an illusory consciousness of movement when the muscles are paralysed and contraction rendered impossible. The latter have as their object to vary the conditions of muscular action, e.g., in exciting muscular contraction artificially by means of a galvanic current, or in producing passive movements, as when another person flexes our arm, and to note the results. These researches, though the results are by no means free from discrepancies, go, on the

1 As brought about in part at least by a re-entering sensory process, muscular sensations are sometimes spoken of as 'reflex' phenomena. They are, however, the exact reverse of reflex action proper. In the latter, sensation occasions movement by means of a central arrangement: in the former, movement gives rise to sensation (partly, at least) by means of a peripheral arrangement.