THE PERCEPTION OF TIME. 635 



behind it which only gradually passes away. (See above, 

 pp. 82-85.) Psychological proof of the same fact is 

 afforded by those ' after-images ' which we perceive when a 

 sensorial stimulus is gone. We may read off peculiarities 

 in an after-image, left by an object on the eye, which we 

 failed to note in the original. We may * hark back ' and 

 take in the meaning of a sound several seconds after it has 

 ceased. Delay for a minute, however, and the echo itself 

 of the clock or the question is mute ; present sensations 

 have banished it beyond recall. With the feeling of the 

 present thing there must at all times mingle the fading echo 

 of all those other things which the j^revious few seconds 

 have supplied. Or, to state it in neural terms, there is at 

 every moment a cumulation of hrain-processes overlapping each 

 other, of ichich the fainter ones are the dying phases of processes 

 ivhich hut shortly previous ivere active in a maximal degree. 

 The AMOUNT OF THE OVERLAPPING determines the feeling of the 

 DURATION OCCUPIED. What EVENTS shall appear to occupy the 

 duration depends on just what processes the overlapping pro- 

 cesses are. We know so little of the intimate nature of the 

 brain's activity that even M'liere a sensation monotonously 

 endures, we cannot say that the earlier moments of it do 



Finally, Prof. Mach makes a suggestion more specific slill. After say- 

 ing very rightly that we have a real sensatwn of time— how otherwise should 

 we identify two entix-el}' different airs as being played in the same ' time ' ? 

 how distinguish in memory the tirst stroke of the clock from the second, 

 unless to each there clove its special time-sensation, which revived with it? 

 —he says " it is probable that this feeling is connected with that organic 

 consumption which is necessarily linked with the production of conscious- 

 ness, and that the time which we feel is probably due to the [mechanical?] 

 work of [WiQ. process of ?] attention. When attention is strained, time seems 

 long; during easy occupation, short, etc. . . . The fatigue of the organ of 

 consciousness, as long as we wake, continually increases, and the work of 

 attention augments as continually. Those impre.s.sions which are conjoined 

 with a greater amount of work of attention appear to us as the later." The 

 apparent relative displacement of certain simultaneous events and certain 

 anachronisms of dreams are held by Mach to be easily explicable as effects 

 of a splitting of the attention between two objects, one of which consumes 

 most of it (Beitriigezur Analyse der Empfiudungen, p. 103 foil.). Mach's 

 theory seems wortliy of being better worked out. It is hard to say now 

 whether he, Ward, and Wundt mean at bottom the same thing or not. The 

 theory advanced in my own text, it will be remarked, does not pretend to 

 bean explanation, but only an elementary statement of the 'law' which 

 makes us aware of time. The Herbartian mythology purports to explain 



