is the greatest illumination of the surface or saturation of the colour is interesting, and not readily explained by either of the current theories of light-sensation.

Before leaving this subject a second paper by Lehmann1 must be noticed, in which he advocates the revolving wheel with black and white sectors for photometric purposes, and a paper by Fischer 2 on the interesting phenomena of the stroboscope or wheel of life'.

2. The Duration of Mental Processes.

The department of research which we have just been considering, that concerned with the relation between the psychical state and the physical stimulus, has been aptly called psychophysics, and it might be well to limit the term to this subject, and not use it as synonymous with physiological and experimental psychology. The term psychometry can, in like manner, be confined to the subject which we are about to take up, the measurement of the duration of mental processes. Psychometry has received abundant attention from astronomers, physicists, physiologists and psychologists; nearly half the researches undertaken in the Leipsic laboratory are concerned with this subject. We are naturally glad to find it possible to apply methods of measurement directly to consciousness; there is no doubt but that mental processes take up time, and that this time can be determined. The measurements thus obtained are not psychophysical, as those which we have been recently considering, but purely psychological. It may be true that we are in some sort measuring the outside of the mind, but the facts obtained, when we learn how long it takes to perceive, to will, to remember, &c., are in themselves of the same interest to the psychologist, as the distances of the stars to the astronomer or atomic weights to the chemist. But, besides the general interest of psychometrical facts as a part of a complete description of the mind, these times are of further and great use to the psychologist, as they help him in analysing complex mental phenomena, and in studying the nature of attention, volition, &c. It should also be noticed that psychometrical experiment has brought, perhaps, the strongest testimony we have to the complete parallelism of physical and mental phenomena; there is scarcely any doubt but

6

1A. Lehmann, "Ueber Photometrie mittelst rotirender Scheiben," iv. 231-240.

20. Fischer, "Psychologische Analyse der stroboskopischen Erschei nungen," iii. 128-156.

that our determinations measure at once the rate of change in the brain and of change in consciousness.

While the importance of psychometrical research for the special student of mind would be admitted, it seems to me that its general interest has been overlooked. Time, like size, is relative. If all things should suddenly move more slowly or more quickly than at present, there would be no change for us. If, however, our physiological movements and mental processes should take place at the same rate as now, while our objective measures of time should move twice as fast, the days of our years would become seven score years, instead of three score years and ten, but we should not for this reason live any the longer or be any the older. If, on the other hand, we should live as many years as at present, but the rate of our physiological and mental motions be doubled, we should live twice as long and become twice as old as now. It would, consequently, be of immense. theoretical and, perhaps, practical importance to learn whether in the course of evolution the molecular arrangement of the nervous system becomes more delicately balanced, so that the physical changes corresponding to our thinking pass more quickly-whether as thoughts become broader, feelings more intense and will stronger, the time they take up becomes less. It is thus an interesting branch of research to determine the time required for the simpler and more complex mental processes, and to study the variation in persons of different race, sex, age, education, occupation, &c.

It will not be necessary to describe at length the psychometrical researches undertaken at Leipsic, as the most recent of these have been printed in MIND. Most of the earlier work on this subject was then reviewed; attention should, however, be called to researches by Kraepelin and by Berger. Kraepelin studied the effects of certain drugs on the duration of a reaction and of simple mental processes. These

8

1 J. McK. Cattell, "The Time taken up by Cerebral Operations," MIND XI. 220-242, 376-392, 524-538; cp. xi. 63-5.

2 M. Friedrich, "Ueber die Apperceptionsdauer bei einfachen und zusammengesetzten Vorstellungen," i. 39-77, ii. 66-72.

M. Trautscholdt, "Experimentelle Untersuchungen über die Association der Vorstellungen," i. 213-250.

E. Tischer, "Ueber die Unterscheidung von Schallstärken," i. 495-542. W. Moldenhauer, “Ueber die einfache Reactionsdauer einer Geruchsempfindung," i. 606-614.

J. Merkel, "Die zeitlichen Verhältnisse der Willensthätigkeit," ii.

73-127.

E. Kraepelin, "Ueber die Einwirkung einiger medicamentöser Stoffe auf die Dauer einfacher psychischer Vorgänge," i. 417-462, 578-605.

times seem to be at first lengthened and then shortened by ether and chloroform, and at first shortened and then lengthened by alcohol, a difference of action which, perhaps, has less to do with different effects of the drugs on the nervous system than with the method of taking them, ether and chloroform being inhaled and alcohol drunk. Berger,1 in experimenting with light, sound and electric shock, found the reaction-time to become shorter as the stimulus was taken stronger. According to these experiments, the reaction-time for the several colours is the same.

It yet remains to notice some unpublished experiments made by L. Lange. He finds that the reaction-time is nearly twice as long when the attention is concentrated on the sense-organ as when it is concentrated on the hand. Wundt looks on these results as important, holding the "muscular" reaction to be reflex, while the "sensorial" includes apperception and volition. I have pointed out that the reaction is at first voluntary, but that with practice the process becomes reflex and the time shorter. We must wait for the publication of Lange's results, and, perhaps, for new experiments, before we know whether an unpractised observer could immediately make his reaction reflex and quicker by concentrating his attention on the movement to be made, or whether the reaction-time of a practised observer would become voluntary and lengthened if he concentrated his attention on the sense-organ.

3. The Time-Sense.

Under this head I shall notice experiments concerned with the time-relations of perceptions and our power of estimating intervals of time. Together with this subject corresponding researches on space might be grouped, but experiments on local signs, sensation - areas, binocular vision, massiveness of sensations, &c., have not as yet been undertaken at Leipsic.

Stimuli must be separated by a certain interval of time in order that they may be recognised as distinct. This is doubtless in many cases a physiological fact due to inertia in the sense-organ. Thus in sight a chemical process is supposed to take place, and this does not reach its maximum until the stimulus has worked some little time, sec. per

1G. O. Berger, "Ueber den Einfluss der Reizstärke auf die Dauer einfacher psychischer Vorgänge mit besonderer Rücksicht auf Lichtreize," iii. 38-93; also Cattell, Brain, vol. viii.

2 MIND Xi. 232.

haps, and continues after the stimulus has ceased. Lightstimuli following each other at intervals shorter than sec. are fused together into one sensation. In the case of sound and of touch the transference from external motion into a nervous impulse seems to be of a more mechanical nature than in the case of sight, and stimuli separated by a shorter interval may be given in consciousness as distinct sensations. The problem becomes more truly psychological when different senses are affected. Exner1 found that the interval between such stimuli must be to sec. before the correct order could be given. Wundt,2 and afterwards v. Tehisch, experimented with an apparatus made so that a pointer passed a scale and when it reached a given division a sound, touch, or electric shock was produced. The problem was to decide what division of the scale the pointer seemed to have reached when the sound was heard or the touch felt. In this experiment there was usually what Wundt calls a "negative displacement," the added stimulus being associated with a position of the pointer earlier than that at which it had in reality been produced. The experiment was varied by altering the rate at which the pointer moved, and by making the added stimulus a complex affecting different senses. The results are perhaps explained by, and in return throw light on, the nature of attention.

Mach and Vierordt 5 first undertook to determine how accurately intervals of time can be compared. This work has been continued in the Leipsic laboratory by four elaborate researches, which, however, do not seem to have given final or satisfactory results. The first three of these need not detain us; we must, however, notice a recent paper by Glass. He finds that times shorter than 2 sec. are overestimated, and those longer than 4 sec. underestimated. He concludes, further, that multiples of 11 sec. are estimated more correctly than other times, and that the "psycho

1 Pflüger's Archiv, xi.

2 Physiologische Psychologie, 2te Auflage, ii. 264 ff.

3 W. v. Tchisch, "Ueber die Zeitverhältnisse der Apperception einfacher und zusammengesetzter Vorstellungen, untersucht mit Hülfe der Complicationsmethode," ii. 603-634.

4 Sitzungsberichte der Wiener Akad., 1865.

5 Der Zeitsinn, Tübingen, 1868.

6 J. Kollert, "Untersuchungen über den Zeitsinn," i. 78-79.

V. Estel, "Neue Versuche über den Zeitsinn," ii. 37-65, 475-482.

M. Mehner, "Zur Lehre vom Zeitsinn," ii. 546-602.

7 R. Glass, "Kritisches und Experimentelles über den Zeitsinn," iv. 423-456.

physical law" holds for the time-sense. Glass's experiments have been carried out with the greatest care, but the fact that four researches on the same subject have all given discordant results, leads us to suppose that something must be wrong in the methods used. Such an error in method is not hard to find. When the experimenter knows that a certain estimate will correspond with the law he has set up, he as a matter of course, though quite unconsciously, makes such estimate. Thus Glass made all his experiments on himself, and the same interval was estimated 100 times in succession. He gives his results in three series; the first of these does not correspond at all with "the law," the second approximates to it, but with considerable irregularity; in the third series he took the intervals where he expected to find his relative maxima and minima, and found them most accurately. The experiments by Stevens1 on the time-sense, contributed to MIND, seem more satisfactory than those from Leipsic.

4. Attention, Memory and the Association of Ideas.

In the course of the experiments which we have been considering we have advanced from the outworks toward the citadel of the mind. We first examined sensation and its relation to the physical and physiological processes which accompany it. While the sensation is a fact of mind in no wise resembling the matter in motion with which it is associated, physical and mental processes have one important characteristic in common, an order in time. We found that the time taken up by mental processes can be measured in much the same way as physical change. We next considered experiments such as are meant to throw light on our spaceand time-sense. We now, in considering attention, memory and the association of ideas, find ourselves at the centre of the mind, and in so far as such subjects are open to experiment, the results are of special interest to the psychologist. As has already been pointed out, our classification of experimental research is a matter of convenience and to some extent artificial. Many of the experiments already noticed concern matters now to be considered; for example, the "least perceptible difference" is a fact of attention, and the reproduction of time-intervals a fact of memory.

There seems to be an upper and a lower limit to consciousness or attention. On the one hand we cannot attend to a presentation of more than a certain degree of complexity; on

1 L. T. Stevens, "On the Time-Sense," MIND Xi. 393-404.