CHAP, in.] SIGHT. 883 



bright light, the sensation almost immediately that it has IM"_MUI 

 suddenly diminishes or even disappears and then is immediately 

 again re-established. 



552. From the prolonged duration of visual sensations it 

 results that when two or more stimuli, such as two or more 

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

 the two sensations or the several successive sensations are fused 

 into one more or less uniform sensation. Thus a luminous point 

 moving rapidly round in a circle gives rise to the sensation of a 

 continuous circle of light. We might, in a very general manner, 

 compare this with the way in which a series of simple muscular 

 contractions resulting from rapidly repeated induction shocks 

 are fused into a fairly uniform tetanus. When the stimuli 

 succeed each other so rapidly that each sensation begins before 

 its predecessor has had time to appreciably decline, the total 

 sensation is as completely uniform as if the stimulus were con- 

 stant. If the interval between each two stimuli be just so long 

 that each sensation in turn has had time to distinctly diminish 

 before the next sensation begins, the result is a " flickering " ; 

 and there are of course many degrees of flickering between a 

 perfectly steady and an obviously intermittent light. The inter- 

 val at which fusion takes place, that is, the interval between 

 successive, stimuli which must be exceeded in order that suc- 

 cessive 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 ^ sec., with a strong light -fa or 

 g^ 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 com- 

 pletely fused, ceases when the rotation becomes so rapid that 

 each pair of black and white sectors takes only -^ sec. in pass- 

 ing before the eye. When a brighter illumination is used the 

 rapidity must be increased before the flickering disappears ; this 

 is owing to the decline of the stronger sensation, as stated above, 

 beginning earlier and being more rapid than that of the weaker 

 sensation. 



553. When a luminous point excites the retina, we recog- 

 nize in the sensation not only the features of intensity, duration 

 and constancy or steadiness, but also a character \vhirh is de- 

 pendent 011 th position in the retina of the image of the lumi- 

 nous point. We recognize the sensation caused by a luminous 

 point whose ima^v falls on the temporal side of the retina, as 

 different and distinct from the sensation caused by a luminous 

 point whose image falls on the nasal siile of the retina, and so 

 with other positions of the images; indeed, as we shall see pres- 

 ently, we are able to distinguish, to recogni/.e as different and 

 distinct, two sensations excited by two luminous points, the 



