THE PHENOMENA OF EXCITATION 



539 



sequently the brightness of the black and white sectors oscillate around a 

 mean value. From a certain rate of rotation onward the whole disk appears 

 uniformly gray, its brightness being the 

 same as would be obtained if all the light re- 

 flected by the white sectors were distributed 

 uniformly over the whole disk. 



Or in the form of a theorem, when a 

 point on the retina is affected in regular 

 periodic succession for a certain time a by 

 rays of a certain intensity, and is entirely 

 unaffected for a certain time 6, if the en- 

 tire period a -{- b is short enough, the sen- 

 sation produced will be a perfectly continu- 

 ous one and of a strength which (within 

 certain limits at least) corresponds to that 

 obtained by continuous stimulation with 

 light rays of an intensity ^ (Talbot's 

 proposition). With light of medium intensity the period a 

 less than 0.04 second. 



FIG. 233. After Helmholtz. 



b need not be 



The time required to reach the maximum excitation of the stimulus, what- 

 ever the interval, is but a fractional part of a second e. g., as shown graphically 

 in Fig. 235, about 0.217 second. Beyond the maximal point, as is evident from 

 the figure, the excitation gradually declines in intensity owing to the onset 

 of fatigue. 



The time required to produce the maximal excitation is different for the 

 different pure colors, being least for the red and greatest for the green (Kuiikel). 



Likewise the time required for the 

 retinal excitation to wear off is dif- 

 ferent for the different colors. 



A. FATIGUE AND RECOVERY 

 OF THE VISUAL ORGAN 



When one looks fixedly for a 

 time (with a light of moderate in- 

 tensity, five to fifteen seconds), at 

 a bright object, and then directs 



citation of the retina successively by black and th t uniforml y illumi . 



white sectors (as m Fig. 233), after Fick. If . J 



m-n, o-p, etc., represent the white sectors, and nated surtace, he perceives on the 

 a-b, c-d, etc., the black, then the progress of latter an after-image, in which the 



excitation is indicated by the broken line. bright parts Q f the ob j ect appea r 



Starting at O, the retina having just been ex- j i j J.-L JT L -LA 



posed to darkness, the excitation rises suddenly dark and the dark P arts bright. 



at first then more slowly ; at n the excitation That is, the image is just the re- 



verse of what we have called a 





 FIG. 234. Schema to illustrate the course of ex- 



ceases suddenly, but requires some time to fall 

 to the zero point, and so on. The more rapid 

 the rotation, the more will the broken line tend 1'^ ax^-^cig^ c*^ , 



to become a horizontal line. scribed as a negative after-image. 



This phenomenon is. due to fa- 

 tigue of some part of the visual organ, in all probability of the retina itself. 

 The bright light falling continuously on a certain point of the retina, fatigues 



