THE SENSE OE VISION. 345 



minated white surface (e. g. a sheet of white paper), we are usually unconscious 

 of any diminution in the intensity of the sensation, but it can be shown that 

 the longer we look at the paper the less brilliant it appears, or, in other words, 

 that the retina really becomes fatigued. To do this it is only necessary to place 

 a disk of black paper on the white surface and to keep the eyes steadily fixed 

 for about half a minute upon the centre of the disk. Upon removing the disk 

 without changing the direction of the eyes a round spot will be seen on the 

 white paper in the place previously occupied by the disk. On this spot the 

 whiteness of the paper will appear much more intense than on the neighboring 

 portion of the sheet, because we are able in this experiment to bring into direct 

 contrast the sensations produced by a given amount of light upon a fresh and 

 a fatigued portion of the retina. 1 



The rapidity with which the retina becomes fatigued varies with the color 

 of the light. Hence when intense white light falls upon the retina, as when 

 we look at the setting sun, its disk seems to undergo changes of color as one 

 or another of the constituents of its light becomes, through fatigue, less and 

 less conspicuous in the combination of rays which produces the sensation of 

 white. 



The After-effect of Stimulation. — The persistence of the sensation after the 

 stimulus has ceased causes very brief illuminations (e. g. by an electric spark) to 

 produce distinct effects. On this phenomenon depends also the above-described 

 method of mixing colors on a revolving disk, since a second color is thrown 

 upon the retina before the impression produced by the first color has had time 

 enough to become sensibly diminished. The interval at which successive stim- 

 ulations must follow each other in order to pro- 

 duce a uniform sensation (a process analogous 

 to the tetanic stimulation of a muscle) may be 

 determined by rotating a disk, such as repre- 

 sented in Figure 156, and ascertaining at what 

 speed the various rings produce a uniform sen- 

 sation of gray. The interval varies with the 

 intensity of the illumination from 0.1" to 

 0.033", and may, therefore, be used as a 

 measure of the intensity, as in the method of 

 u flicker photometry." 2 The special advan- 

 tage of this method is that it affords a means Era. m-Msk to illustrate the persistence 



. . ... . ... of retinal sensation (Helmholtz). 



01 determining the relative intensity of lights 



of different colors. The duration of the after-effect depends also upon the 

 length of the stimulation and upon the color of the light producing it. the 

 most persistent effect being produced by the red rays. In this connection it 

 is interesting to note that while with the rapidly vibrating blue rays a less 



1 Although the retina is here spoken of as the portion of the visual apparatus subject to 

 fatigue, it should be borne in mind that we cannot, in tin- present state of our knowledge, dis- 

 criminate between retinal fatigue and exhaustion of tin- visual nerve-centres. 



2 Rood : American Journal of Science, Sept., 1893. 



