and the Intensities of Lights of different Colors. 13 



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 color 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 \ and } 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 color or a 

 circle or triangle has been presented to our vision. 



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

 experiments with the electric flash lasted about J of a second, 

 yet the most careful scrutiny could detect no change in sensa- 

 tion at and immediately following the flash. The contrast- 

 colors, 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 to fall, falling with two 

 oscillations in intensity, so that all the events of the phenome- 

 non take place in about \ of a second. However, no vague 

 impression of surfaces merely differing in illumination and 

 then suddenly changing into a color and its contrast-color 

 could be detected. I think that this interval of no color sensa- 

 tion, if it exist, must be of exceedingly short duration ; but 

 such a period of light without color cannot be detected, and if 

 it cannot be perceived, then, so far as we are concerned, it 

 appears to me, that there can be no hesitation in the percep- 

 tion of the colors, and no " fluctuation of the judgment " and 

 " dividing between two images the difference in color which 

 really exists " before the mind reaches its conclusion as to the 

 character of the colors. 



The following experiments were separately made on three 

 persons between whom no communication had passed as to the 

 nature of the experiments to be tried on them. I placed a 

 gray ring on a ultramarine disk in front of the Holtz-machine 

 and requested the observer, who had implicit confidence in my 

 truthfulness, to describe to me as accurately as possible the 

 exact hue of the pink, or rose color, or red he would see on a 

 green ground at the instant of the electric flash. Each observer 

 at once said : " it is not pink, the 'ring appears yellow on a blue 

 ground." Xow in each of these experiments the observer was 

 prepared, by my pardonable lying, to see red on a green 

 ground, and to see yellow on a blue ground his mental con- 

 dition of anticipation to see red on a green ground was first 



