RECUKRENT VISION 91 



the intensity of the stimulus, and the rate of diminution of the action- 

 time with increase of intensity of the light diminishes as the intensity 

 increases, so that with light of ordinary, fairly high, intensity it is very 

 small. It has been pointed out above that when the duration of stimula- 

 tion is very short the response is a series of pulses of sensation. If the 

 stimulus be other than very brief the sensation shows no such pulses. 

 McDougall has shown that with a light whose action-time is 

 61'^(yfi^7y sec.) the multiple character of the response persists until the 

 duration of its action exceeds its action-time by about IG*^, and ceases 

 altogether when the duration of the action of the light exceeds its 

 action-time by about 40"^. As regards the variation of the intensity 

 of the sensation with the duration of the stimulus when that duration 

 is less than the action-time McDougall concludes that it follows the 

 "photographic law," i.e., for such duration the intensity of the sensation 

 varies directly with the duration of the action of the light. In mathematical 

 terms the law may be stated thus : Sec I . St, where S is the intensity 

 of the sensation, I the intensity of the stimulus, and 8^ a small increment 

 of time. This law is a corollary of the Talbot-Plateau law {vide infra), 

 and was assumed by Talbot. With very dim lights of scotopic value 

 the action-time is probably slightly greater than 200"^ or i sec. The 

 practical conclusion — that the dimmest light visible to the peripheral 

 retina of the dark-adapted eye, i.e., the dimmest light perceptible under 

 the most favourable conditions, must be allowed to act for a period 

 of not less than 200"^ or i sec. in order to be perceptible — is of great 

 importance in the construction of lighthouse flash-lights and so on. 

 Its practical importance is increased, as well as the theoretical signifi- 

 cance, by McDougall's observations which show that the action-times 

 of red, green and blue lights are the same for lights of equal intensities, 

 i.e., intensities which excite sensations of equal luminosities. 



Briickner and Kirsch^ have investigated the chromatic action-time 

 or the minimum time during which a coloured light must stimulate 

 the retina in order that the colour may be discriminated {Farbenzeit- 

 schwelle or specifische Zeitschivelle). They found that the time varies 

 with the intensity of the white light acting upon the retina both before 

 and after stimulation with the coloured light and is nearly proportional 

 to the brightness of this " grey " sensation. The chromatic action-time 

 follows Weber's law, at any rate within certain limits. It varies 

 inversely as the width of the pupil and the size of the retinal area 

 stimulated, but not proportionally. The intensity of the following 



^ Ztsch.f. SinnespTiysiol. xlvi. 229, 1911. 



