In the following account of some of the most interesting of recent physiological experiments with the senses, I propose to select simply those which bear directly on one of these two results: serving either to render more clear and precise the nature and laws of sensation, or to illustrate and confirm some general mental principle.

Perhaps the most striking of these experiments are those which aim at a more exact measurement of sensation. A number of physiologists, chiefly German, have occupied themselves with the objective mode of measuring the sensibilities already alluded to; and although some of the results of these investigations appear to add little to our knowledge of the general relation between nervous stimulation and conscious sensation, they are, perhaps, worth recording as data, by help of which such more general principles are to be arrived at.

With respect to the duration of a sensation, viewed as the effect of a nervous process, there are several points deserving attention. First of all, it is clear that the initial physical change occupies an appreciable time. It has been estimated that when a muscle is made to contract by communicating a short electric stimulus to the motor nerve, about one-sixtieth of a second elapses before the effect of contraction becomes visible. Yet there presents itself as yet no method of estimating the interval between the application of an external stimulus and the commencement of the resulting feeling. There are two distinct questions involved in this unsolved problem. The first relates to the transmission of a nervous impulse from the periphery to the sentient centres. This point has but little psychological interest. The second refers to the minimum duration of the nervous process in the central regions in order that a distinct sensation may result. It is very probable that there exists some limit of duration, below which a nervous change fails to produce a sensation, and it is supposable that at all times a vast number of such faint fugitive pulsations are passing through the brain, without contributing to consciousness any of its distinct elements. The number of vague transient feelings which fill up the interstices of our clear conscious life, may probably be interpreted as the immature products of too rapidly ceasing pulsations in the nervous substance. Any advance towards the proof and measurement of this minimum interval would be of great value in helping one to determine the minimum duration of a definite and recognisable sensation. Such a discovery, if possible at all, could only be made by the help of just such objective experimentation as that already accomplished. The

physical processes taking place in the hidden structures of the brain lend themselves to no immediate observation, and can only be approximately determined as the intervening stages between an observable stimulation at the periphery of a sentient nerve and a discoverable effect in the subject's mind.

If it be as yet impossible to measure the velocity of an inward nervous current, a good deal has been done to determine the duration of such a current in the central regions. It is well known that a nervous change lasts considerably longer than the contact of the external stimulus which sets it agoing; and this fact is of great significance in the explanation of the ideal reappearances of the sensation. The existence of this self-prolonged sensation is best observed in the impressions of the eye, and it is here that the phenomenon has received the most precise estimation.

Students of optics are familiar with the fact that when a circular disc, having alternate black and white sectors, is made to rotate about an axis, there is a certain rate of rotation above which all single impressions of the black and white surfaces cease, giving place to a continuous sensation of grey. This is at once accounted for by the nervous law just referred to, that movements in the fibres and grey substance produced by external stimulation outlive this stimulation by a certain brief interval. The impression left at any given point of the retina by a white sector continues unabated during the brief interval in which the black sector passes over it, and the effect is the same as if the quantity of light issuing from the white sectors were distributed uniformly over the whole surface of the disc. By ascertaining the time occupied by each rotation, and the fraction of the whole circumference represented by the width of the black sector, it is possible to measure the exact maximum duration of an impression of light in its unabated intensity. The physiologists who have attempted this experiment have reached slightly different results. Thus Plateau found that in ordinary daylight the time of transition of a black sector, and so of the unchanged impression of light, could be made as large as 191 seconds. Professor Helmholtz, again, says that with strong lamplight the time of transition must not be greater than th of a second, though in weak moonlight it may be as much as th. It should be added that when the light

48

* See his Handbuch der physiologischen Optik. Part II. § 22. Die Dauer der Lichtempfindung. This work, besides being a great original contribution to the subject, gives very full accounts of previous physiological investigations and theories dating from the earliest known discoveries.

is weaker the time of the unchanged after-impression is greater, there being here less exhaustion of the nerve by the successive stimulations. Further, rays of different colour appear to produce after-effects of unequal durations, an impression of blue remaining longer than one of red or yellow. This fact is susceptible of a peculiar interpretation on the hypothesis of Thomas Young as revived by Helmholtz. According to this, there are three classes of fibres distributed through the retina, sensitive to red, blue, and green rays respectively. From this it would follow that the fibres sensitive to blue are most susceptible of the after-effect we are now considering, their molecular movements being in their nature the most prolonged. Again, Plateau sought to determine the time during which this after-image (Nachbild) continues in decreasing intensity. This was found to be greater as the light was stronger. From this it follows that a powerful stimulus of light produces an after-impression which begins to fade much sooner than that of a feeble stimulus, though it has on the whole the longest effect. These same experiments with the discs show, too, that the after-impression of a light-stimulus depends simply on the quantity of light falling on a given point of the retina, so that it is the same thing whether an intense light acts for a brief interval or a faint light for a longer interval.

Another interesting point in connection with the duration of sensation, is the time required for exhausting a nerve. When, for example, a fibre of the optic nerve has been stimulated for some while, there presents itself, instead of the positive effect just considered, a negative one, namely a temporary diminution of the nerve's sensibility. This fact accounts for the phenomena of negative after-images. Helmholtz tells us that in order to experience these in greatest distinctness and for the longest time, it is best to let the first stimulating light act for a period of five to ten seconds. In this case the positive after-effect is evanescent and inappreciable. He found that, under these circumstances, a negative image of some bright clouds remained as long as eight minutes. It has been established, further, that rays of different colour exhaust the optic nerve with unequal rapidity. According to Young's theory, the complementary image that succeeds a long impression of a given colour arises from the temporary incapacity of the corresponding class of nerve fibres. Thus, after looking some time at a green object, the fibres sensitive to green rays become exhausted, so that when the eye is afterwards directed to a white surface, the part of

the retina which received the green rays is unaffected by the corresponding ingredients in the white light, and so the resulting sensation is that produced by the remaining rays, namely, the complementary impression of purple. This negative effect on the nervous substance seems, like the positive one already described, to have a different duration in the different classes of fibres. An impression of white light, as afforded by the sun, may sometimes leave a series of images of various colours, and this phenomenon is probably due both to the various susceptibility of the three classes of fibres to a positive after-impression, and to their unequal liability to exhaustion. At the same time it seems impossible, according to Helmholtz, to assign proportions to the two influences in this compound result.

Although the phenomena of self-sustaining nervous activity and of temporary exhaustion seem only susceptible of accurate observation in the region of visual impression, they pretty certainly extend to all departments of sensation. Thus, it is very easy to observe at times a lingering after-sensation of tone left by some external stimulus, though I am not aware that this special nervous susceptibility has been similarly investigated.

One or two questions present themselves in connection with these data which can hardly fail to interest the psychologist as well as the physiologist. For example, is the rapid obliteration of the afterimpression simply one aspect of nervous exhaustibility? It seems presumable, on antecedent grounds, that nervous fibres which are but slightly retentive of this after-effect should be greatly liable to exhaustion and the need of periodic repose. Helmholtz seems to think there is a connection between the two in the region of visual sensation, though he says (p. 383) we do not know the law of their dependence. Comparing the nerves of the different senses, one would be inclined to say that those which display most conspicuously the capacity for after-impression, pre-eminently the optic nerve, retain their sensibility longest under the action of a uniform stimulus.

Again, it might be asked whether the degree of persistence of the after-sensation bears any constant relation to the degree of the sensation's susceptibility of ideal reproduction. The impressions of the eye, which manifest most conspicuously the first quality, are also among the most easily recovered of our sensations. Further, it has been observed by Purkinje and Aubert, that an impression of light fades away much more quickly on the peripheral parts of the

retina than at the centre, and it is clear that our visual recollections consist almost exclusively of ideas of impressions projected on the central region of perfect vision. Hence it is just possible that a more exact method of estimating the duration of this later effect of stimulation in the other sensations would bring to light a simple law of co-variation between the two. It may be presumed, too, à priori, that since these positive after-images probably imply a selfsustained activity in the related parts of the centres after the peripheral stimulation has ceased, they will involve, as a further consequence, a liability to central activity without any such peripheral initiation, that is to say, the physical process which underlies the ideal revival of the sensation. The liability of the nerves to temporary exhaustion appears, still more clearly than susceptibility to after-impression, to be a universal property of the cerebro-spinal system, constituting the physiological basis of the well-known psychological fact, that conscious life is a continual transition. At the same time, this need of relief does not present itself in precisely the same forms in all modes of nervous action. As we have seen in the case of visual impressions, the fibres sensitive to one variety of coloured light are much sooner exhausted than those which subserve another kind of sensation. It would be interesting to inquire whether the demand for temporary cessation is not greater in the case of those parts of the nervous system least employed in daily life. Apart from that increased facility of the directing muscular action which results from the frequent repetition of any impression, it is quite conceivable that those sentient fibres which are most frequently used, come to possess an increased capacity for unbroken work. If, as we have seen to be probable, great liability to exhaustion goes with feeble tenacity of after-impression, the fact just now mentioned, that the peripheral parts of the retina display the lowest degree of sensational persistence, would appear to favour this view of the effects of exercise on the working capacity of a nerve. *

Finally, it may be hinted that if the force of resistance to ex

On the Evolution Hypothesis it might be possible to explain any innate inequality in this respect, for example between the optic and the gustatory nerve, by supposing them to be the effects of long processes of exercise through many generations. It seems plausible, indeed, that both the properties of nervous fibre here spoken of, the capacities for prolonged stimulation and for post-stimulative action, are gradually strengthened with the functional development of nervous tissue.