THE SENSES 819 



and close. Like the muscle, the retina seems to possess a store of 

 explosive material which the stimulus serves only to fire off. The 

 retina, like the muscle, is exhausted by its activity, and recovers 

 during rest. Like the muscle curve, the curve of retinal excitation 

 rises not abruptly, but with a measurable slowness to its height, and 

 when stimulation is stopped, takes a sensible time to fall again, the 

 retinal impression outlasting the luminous stimulus by about one- 

 eighth of a second. With comparatively slow intermittent stimuli 

 the retinal, like the muscle curve, flickers up and down. When the 

 rate of stimulation is increased, the steady contraction of the tetanized 

 muscle is analogous to the fusion of the individual stimuli by the 

 tetanized retina (or retino-cerebral apparatus) into a continuous sensa- 

 tion of light, such, e.g., as the bright * trail ' of a falling star, or the 

 fiery circle traced in the air when a firebrand is rapidly whirled round. 

 But the maximum retinal excitation which a stimulus of given 

 strength can call forth depends much more closely upon the time 

 during which the stimulus acts than the maximum contraction does 

 upon the length of the muscular stimulus. 



As the strength of the light increases in geometrical progression, 

 the time during which it must act in order to produce its maximum 

 effect decreases approximately in arithmetical progression (Exner). 

 For light of moderate intensity this time is about J second. Since 

 for complete fusion the stimuli must follow each other at a much 

 more rapid rate than four in the second, the intensity of the resultant 

 sensation is always less when a succession of similar stimuli are fused 

 than when one of the stimuli is allowed to produce its maximum effect. 



If the time of each stimulus is equal to the interval during which 

 there is no stimulation, the sensation, when complete fusion has been 

 reached, is the same as would be produced by a constant light of 

 half the strength employed. And, in general, if m be the pro- 

 portion of the time during which the eye is stimulated by a light of 

 intensity /, and n the proportion of the time during which it is not 

 stimulated, the resultant impression is the same as that which would 



be produced by an uninterrupted light of intensity ( )/. This 



\m + n) 



is Talbot's law, which may be expressed without the aid of symbols 

 thus : When a light of given intensity is 

 allowed to act on the eye at intervals so short 

 that the impressions are completely fused, the 

 resultant sensation is independent of the abso- 

 lute length of each flash, and is proportional 

 only to the fraction of the whole time which is 

 occupied by flashes and to the intensity of the 

 light. Talbot's law may be readily demon- 

 strated by means of a rotating disc with 

 alternate white and black sectors (Fig. 315), 

 so arranged that the same proportion of the 

 circumference of each of the three concentric 

 zones is black. LAW. 



When the rotation is sufficiently rapid to 

 give complete fusion (say 20 to 30 times a second), the whole disc 



522 



