520 VISUAL SENSATIONS. [BOOK in. 



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 statements. 



The sensation has a duration much greater than that of the 

 stimulus, and in this respect is comparable to a muscular contrac- 

 tion 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 -fa sec., with a strong light ^ or -J^ sec. This may be shewn 

 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 be- 

 comes so rapid that each pair of black and white sectors takes 

 only j 1 ^ sec. in passing before the eye. When a brighter illumina- 

 tion is used the rapidity must be increased before the flickering 

 disappears. That part of the sensation which is recognised 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 

 colours, being shortest with yellow, intermediate with red, and 

 longest with blue. 



The duration of a stimulus necessary to call forth a sonsation is 

 exceedingly 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 posi- 

 tion. When a moving body is illuminated by several rapid flashes 

 in succession, several distinct images corresponding to the positions 

 of the body during the several flashes are generated : the images of 

 the body corresponding to the several flashes fall on different parts 

 of the retina. 



