138 EEPORT — 1876. 



of hearing and vision hare been collected ; and most of tliese facts, inasmucli as 

 they assist him in understanding the conditions of sensory impressions and sensa- 

 tional effects, are of importance to the psychologist. 



Measurement of TniE is Sensoby Imphessions. 



The next step of importance made by physiology into the domains of psychology 

 is the measm-ement of time or dm-ation in sensational effects*. This has been 

 carefully measured by objective methods. Speaking generally, the time occupied 

 from the commencement of the action of the stimidus to the termination of a 

 sensation may be divided into four portions, each of which has a certain psycholo- 

 gical interest : — First, an interval of time is occupied by the primary physical change 

 produced by the stimulus. Duriug this interval, called the period of latent stimu- 

 lation, no effect is observed. Thus, when a motor nerve distributed to a muscle is 

 stimulated by a short electrical shock, about l-60th of a second passes before the 

 muscle contracts. Second, when the change in the nerve or terminal organ has 

 begim, a second interval of time is occupied in the transmission of the impression 

 to the nerve-centre, which is svicceeded by a third interval, dming which changes 

 occur in the nerve-centre, and the result of which is a sensation. The time occu- 

 pied in transmission, or the rate of conductivity in nerve, is tolerably well kuoTsni, 

 being at the rate of about 200 feet per second in the nerves of men ; but the time 

 occupied in the production of the sensation in the centre has not yet been clearly 

 ascei-tained, owing to the difficulty of supposing such a sensory nerve-centre to be, 

 previous to the stimidus, in a state of absolute inaction. Lastly, it has been fomid 

 that when a neiTous action of any kind has been initiated by a stimulus, it goes on 

 for some time after the stimidus has ceased to act. This prolongation of the sen- 

 sation may be well studied in the case of impressions on the eye, where the time of 

 the duration of the impression has been measured by Ilelmholtz, Plateau, and 

 others. These distinguislied observers also found that the length of time occupied 

 by the after-effect varied according to the intensity of the light. Thus after a 

 weak light, the unchanged impression lasts longer than with a strong light. A 

 strong illumination is followed by an after impression fading sooner than with a 

 feeble stimulus — the result being that, so far as the retina is concerned, it comes to 

 the same thing whether an intense light acts for a brief time, or a faint light for a 

 longer time. 



Exhaustion or NER\rE on Sensory Organ, 



This line of research has also made it possible to measure the time required for 

 exhausting a nerve or sensory organ. When, for 'instance, a limited area of the 

 retina has been stimidated for a certain time, and the stimulus has been removed, 

 the after positive effect, due to increased excitation of the parts, disappears, and is 

 followed by a negative effect, due to temporary diminution of the sensioility of the 

 parts, in the form of what is called the negative after-image. Suppose, for ex- 

 ample, an area of the retina be acted upon for a period of from five to ten seconds, 

 and the stimulus be then removed, tne so-called positive after-image vaioishes 

 quickly, and the negative after-image, frequently ot a complementary colour to 

 that of the exciting cause, appears, and lasts for a short time, gradually fadmg 

 awav as the nervous parts recover from the effects of the stimulus. Similar pheno- 

 mena maj^ be observed in studying the durations of sensations of tone, which I 

 have frequently perceived in experiments made by myself; but it is more difficult 

 to ideutif}', by description and designation, the after-effects in the case of audition 

 than in the case of -s ision . Probably it may be found still more difficult to notice 

 these after sensations in the other senses, although in all there is often the experi- 

 ence of a lingering feeling after the cause has been removed, which no doubt has 

 its place in those transient sensations which assist in filling up the spaces, as it 

 were, in our conscious life, 



In experiments upon a sensory organ, such as the retiua, a little cousideralion 



* In tbo following obser\ations I am much indebted to the essays of Mr. James Sully, 

 contained in his volume, ' Seniation and Intuition ' (London, 1874). 



