552 THE NERVOUS SYSTEM. 



tion in the brain considerably more. It follows from this that, at the 

 instant when the electric spark is seen, it has, in fact, already come 

 to an end ; the interval which elapses before it is perceived by the 

 observer being very much greater than its actual duration. 



This accounts for a peculiar effect, often observed under the use of 

 the electric spark, namely : that bodies in rapid motion, when illumi- 

 nated by an instantaneous discharge, appear as if at rest. A disk with 

 black and white sectors, in revolution under continuous light, appears 

 of a uniform gray. But if such a disk, revolving in a dark room, be 

 illuminated by the electric spark, it becomes visible for an instant, with 

 its sectors as distinct from each other as if they were at rest. A jet 

 of water, flowing from a narrow orifice, is transparent in its upper part, 

 but turbid lower down ; and by instantaneous illumination the turbid 

 portion is seen to be composed of sepante drops, which appear motion- 

 less. The passage of a cannon-ball or a bullet by daylight is imper- 

 ceptible ; because it does not rema'n long enough at any one point to 

 efface the persistent impression of objects behind it, But if such a mis- 

 sile should happen to be passing in front of the observer in the night 

 during a thunder-storm, at the moment of a flash, it would be equally 

 visible with other objects, and would appear as if suspended motionless 

 in the air. 



The momentary closure of the eyes in winking, for the same reason, 

 is unnoticed, and causes no interference with sight ; since the visual 

 impression of external objects continues unimpaired during the interval 

 occupied by the movement of the lids. 



The sensibility of the retina is diminished by continued impressions. 

 This diminution seems to take place from the very commencement of 

 a visual impression, so that it may be perceptible within a few seconds. 

 When the image of the retinal blood-vessels is made apparent by chang- 

 ing the position of their shadows (page 530) their figures are visible 

 for an instant with extreme sharpness. But they at once begin to fade 

 and soon become imperceptible. The portions of the retina under full 

 illumination have their sensibility so rapidly diminished, that the 

 shadow, if motionless, is no longer visible by contrast. Those in 

 shadow, on the other hand, become more sensitive by repose; and 

 when the shifting of the light brings them again into illumination, 

 they are already more susceptible to its influence. 



If one eye be covered by a dark glass, and the other used alone for 

 reading or writing, at the end of an hour the difference in retinal sen- 

 sibility of the two will be very apparent. A faintly luminous object 

 in a dark room may be almost imperceptible to the eye which has been 

 in use, while appearing to the other quite brilliant. But this condition 

 is transitory ; and by covering the eye previously in use, and reading 

 or writing with the other, the fatigued organ recovers its sensibility, 

 and that which was before the most sensitive becomes less so. 



The diminution and recovery of retinal sensibility, under excitement 

 and repose, is connected with the phenomena of negative images. 



