76 FUSION OF VISUAL SENSATIONS. [BOOK in. 



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 prede- 

 cessor has had time to appreciably decline, the total sensation 

 is as completely uniform as if the stimulus were constant. 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 per- 

 fectly steady and an obviously intermittent light. 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 

 -g 1 ^ 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 completely 

 fused, ceases when the rotation becomes so rapid that each pair 

 of black and white sectors takes only ^ sec. in passing 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. 



750. When a luminous point excites the retina, we recognize 

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

 constancy or steadiness, but also a character which is dependent 

 on the position in the retina of the image of the luminous point. 

 We recognize the sensation caused by a luminous point whose image 

 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 side of the retina, and so with other positions of the images ; 

 indeed, as we shall see presently, we are able to distinguish, to 

 recognize as different and distinct, two sensations excited by two 

 luminous points, the images of which lie very close indeed to each 

 other on the retina. We distinguish the sensations, however, not 

 by reference to the parts of the retina affected, but by reference 

 to the relations in space of the luminous points giving rise to the 

 sensations. Since this is a matter of some importance we may 

 treat of it in some detail. 



In the vast majority of cases the changes in the retina which 

 give rise to visual impulses, and so to visual sensations, are 

 brought about by light falling on the retina. But the retina 

 may be stimulated by other agencies than that of light. 

 When this is the case the changes in the retina, however 

 produced, if they are able to affect consciousness at all, give 



