D URA TION OF STIMUL US AND SENS A TION 1067 



While there is now no doubt that the general phenomenon of contrast 

 may be referred to a process of physiological induction, there are certain 

 features of contrast which are at present wholly unintelligible physio- 

 logically, and these can only be expressed as facts in psychological 

 terms. Thus, the obscuration of slight differences of colour or brightness in 

 two objects by other differences, such as inequalities of surface and differences 

 of texture, can only at present be given as a psychological fact. Waller l has 

 described an experiment, in which, when a coloured surface is placed on either 

 side of the middle third of a grey strip, the contrast colour in the strip is 

 barely perceptible. If, now, the middle third is marked off by placing a pin 

 across the strip at each end, the contrast colour appears distinctly. One can 

 only give this as an example of the psychological fact that colour contrast is 

 favoured when the induced surface is sharply marked off from surrounding 

 parts. Sherrington 2 has pointed out that individuals may see a difference of 

 colour between two surfaces, due to contrast, until they are convinced that the 

 two surfaces are objectively alike, and may then fail any longer to see the 

 difference in colour. In this, as in other cases, the psychological factors tend 

 to mask the appearances which depend on the physiological reciprocity of 

 adjoining retinal areas. 



The Duration of Stimulus and Sensation. 



Duration and sensibility. — In the section on after-image the 

 factor " duration " has been to some extent considered. In this case, 

 however, attention was limited to the altered condition of sensibility 

 which remains for considerable periods after retinal stimulation. In this 

 and the following sections facts will be considered which illustrate the 

 relation of the duration of the stimulus to the duration of the sensation. 

 However short the duration of the stimulus, the sensation which arises 

 lasts for an appreciable time, and this phenomenon may be termed 

 persistence of vision. It has also been termed positive after-image, 

 but it would probably be better to limit the term " after-image " to those 

 more or less prolonged manifestations dealt with in the previous sections. 



It has been found that the threshold of sensibility depends upon the 

 duration of the stimulus within certain limits, i.e. for durations between 

 '002 and T25 sec. (Charpentier). 3 A feeble light of short duration, which 

 is below the threshold of stimulation, may become visible by increasing its 

 duration ; the duration necessary within the limits mentioned being 

 inversely proportional to the intensity, so that doubling duration has 

 the same effect on sensibility as doubling intensity. For these short 

 intervals the effects of light on the retina appear to accumulate as 

 they do on a photographic plate. The same inverse relation also holds 

 between intensity and extent of retina stimulated (p. 1055), so that 

 summation of feeble stimuli seems to occur both in time and space, 

 and, according to Charpentier, for a given sensation a certain amount 

 of light is necessary, and within the limits it matters little whether this 

 amount is extended over a larger or smaller area, or reaches the eye in a 

 longer or shorter time. Adaptation of the retina to the dark shortens 

 the duration necessary for a sensation in the same way that it lowers the 

 threshold. According to Charpentier, with very short duration coloured 

 light is seen as colourless, i.e. for coloured light there is an interval 

 between the absolute and chromatic thresholds of duration, but this has 



1 Journ. Physiol., Cambridge and London, 1891, vol. xii. p. xliv. 



2 Ibid., 1897, vol. xxi. p. 41. 



3 Arch. d'ophL, Paris, 1890, tome x. p. 108. 



