THE SENSES 937 



cannot be concerned in colour vision, or, at least, they cannot be 

 essential to it, for in the human retina they do not exist. 



The yellow pigment of the macula lutea docs not belong to the 

 layer of rods and cones ; it only exists in the external molecular layer 

 and the layers in front of it ; in the fovea centralis it is absent. 



Time necessary for Excitation of the Retina by Light Fusion of 

 Stimuli. ^Whatever the exact nature of retinal excitation may be, 

 it is called forth by exceedingly slight stimuli. A lightning flash, 



although it may last only - th of a second, lasts long enough 



9 1,000,000 



to be seen. A beam of light thrown from a rotating mirror on the 



eye stimulates when it only acts for 5 th of a second. The 



8,000,000 



minimum stimulus in the form of green light corresponds, as we have 

 already seen (p. 681), to a quantity of work equivalent to no more 



than g erg that is, about ^ gramme-millimetre, or ^ milli- 

 gramme-millimetre, which is the work done by th of a 



10,000,000 



milligramme in falling through a millimetre ; and it cannot be 

 doubted that a portion even of this Lilliputian bombardment is 

 wasted as heat. So quickly, too, is the stimulus followed by the 

 response that no latent period has as yet ever been measured. It is 

 certain, however, that there is a latent period, as surely as there 

 is a latent period in the excitation of a naked nerve -trunk, 

 although this also has never been experimentally detected. The 

 analogies, in fact, between a muscular contraction and a retinal 

 excitation are numerous 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 v ith 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 compara- 

 tively 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 sensation 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 ^ 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 



