of Simultaneous Contrast- Colour. 165 



ception of the colours was much less than ^ of a second. 

 Indeed, on viewing the flash and the illuminated surfaces at 

 the same time, or, hearing the discharge and viewing only the 

 illuminated ring, no interval could be detected by this mode 

 of observation as existing between the instant of the flash and 

 the perception of the colours, and we certainly could have 

 detected a shorter interval than ^ of a second had it existed. 



Incidentally, I will here state that when different geome- 

 trical or irregular figures cut out of white paper and placed 

 on an ivory-black ground were illuminated by the electric 

 flash, the observer, so far as I could ascertain, formed con- 

 clusions as to the forms of the pieces of paper in an interval 

 of time less than the ^ of a second. Certainly the interval 

 of time required to reach a conclusion as to the contrast-colour 

 was less than the ^ of a second. Professor Mendenhall 

 in 1871 (Arner. Journ. Sci.) gives 0*292 sec. for the time 

 required by an observer to respond, by means of an electric 

 chronograph, to the appearance of a white card. When the 

 observer responds by touching the circuit-key with one hand 

 when a white card appears, and with the other hand when a 

 red card appears, Professor Mendenhall finds 0*443 sec. as 

 the time required for the response. Subtracting the first 

 number from the last he obtains '151, or | sec, as the time 

 to form a judgment between white and red. Similar experi- 

 ments gave him 0*202, or £- sec, as the time to judge whether 

 a circle or a triangle appeared to the observer. The inter- 

 vention of the operation of brain, nerve, and muscle before 

 the response is recorded involves complex operations, and the 

 action of right or left sides of brain when the two hands are 

 used. It appears that the plan of eliminating these actions, 

 and thus obtaining a residual time which is attributed to the 

 interval required to form a judgment of colour or of form, is 

 faulty. I am sure that if estimation of the time required for 

 such mental operations is made by the continuous indications 

 of the beats of forks, such intervals as J and J of a second 

 will at once appear much in excess of the time respectively 

 required to form a conclusion as to whether a white or red 

 colour or a circle or triangle has been presented to our vision. 



It is here to be noted that although the after-images of the 

 contrast-colours seen in the light of the electric flash lasted 

 about \ of a second, yet the most careful scrutiny could detect 

 no change in sensation at and immediately following the flash. 

 The contrast-colours, so far as I and others observed, appeared 

 at the moment of the flash. After the instant the image of 

 the flash is formed on the retina there exists, no doubt, an 

 interval of time before we are conscious of the stimulus, 

 whose effects are seen rapidly to rise and then more gradually 



