766 SIGHT 



appear somewhat complicated by the appearance now of positive, now of 

 negative, variations ; but this fact comes out clearly, that the incidence of 

 light on the irritable retina develops an electric change, the magnitude of 

 which is to a certain extent proportionate to the intensity of the light acting 

 as a stimulus. The changes accordingly diminish and cease to appear as 

 the retina gradually loses its irritability after death. We may add that 

 these electric phenomena appear to be quite independent of the condition 

 of the visual purple. 



Simple Sensations. 



657. Relations of the sensation to the stimulus. If we put aside for the 

 present all questions of color, we may say that light, viewed as a stimulus 

 affecting the retina, varies in intensity, that is, in the energy of the luminous 

 vibrations as manifested by their amplitude, and in duration, that is, in the 

 length of time a succession of waves continues to fall upon the retina. The 

 effect of the light will also depend on the extent of retinal surface exposed 

 to the luminous vibrations at the same time. Taking a luminous point, in 

 order to eliminate the latter circumstance, we may make the following state- 

 ments : 



The sensation has a duration much greater than that of the stimulus, and 

 in this respect is comparable to a muscular contraction caused by such a 

 stimulus as a single induction-shock. The sensation of a flash of light, for 

 instance, lasts for a much longer time than that during which luminous 

 vibrations are falling on the retina. Hence, when two stimuli, such as two 

 flashes of light, follow each other at a sufficiently short interval, the two 

 sensations are fused into one ; and a luminous point moving rapidly round 

 in a circle gives rise to the sensation of a continuous circle of light. This 

 again is quite comparable to muscular tetanus. The interval at which fusion 

 takes place, that is, the interval between successive stimuli which must be 

 exceeded in order that successive distinct sensations may be produced, varies 

 according to the intensity of the light, being shorter with the stronger light ; 

 with a faint light it is about T V second, with a strong light ^ or -fa second. 

 This may be shown by rotating rapidly before the eye a disc arranged with 

 alternate black and white sectors of equal width. With a faint illumination 

 the flickering, indicative of the successive sensations from the white sectors 

 not being completely fused, ceases when the rotation becomes so rapid that 

 each pair of black and white sectors takes only ^Q second in passing before 

 the eye. When a brighter illumination is used the rapidity must be in- 

 creased before the flickering disappears. That part of the sensation which 

 is recognized as lasting after the cessation of the stimulus is frequently 

 spoken of as the " after-image." 



Though the duration of the after-image is longer with the stronger light 

 (that caused by looking even momentarily at the sun lasting for some time), 

 the commencement of the decline of the sensation begins relatively earlier, 

 hence the greater difficulty in the complete fusion of successive sensations 

 with the stronger light. The interval at which fusion takes place differs 

 with different colors, being shortest with yellow 7 , intermediate with red, and 

 longest with blue. 



The duration of a stimulus necessary to call forth a sensation is exceed- 

 ingly short ; thus the shortest possible flash, such as that of an electric 

 spark, gives rise to a sensation of light. 



Objects in motion when illuminated by a single electric spark appear 

 motionless, the stimulus of the light reflected from them ceasing before they 

 can make an appreciable change in their position. When a moving body is 



