646 THE -SENSES. 



taneous illumination the turbid portion is seen to be composed of 

 separate drops, which appear to be motionless. A flash of lightning has 

 a similar effect in exhibiting objects which are in motion as if they were 

 quiescent. The passage of a cannon ball or a rifle bullet by daylight 

 is imperceptible ; because, as an opaque object, it does not remain long 

 enough at any one point to efface the persistent impression of the objects 

 visible behind it, and the sight of these objects accordingly does not 

 appear to have suffered any interruption. But if such a missile should 

 happen to be passing in front of the observer in the night time during 

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

 with the other parts of the landscape, and would appear as a motionless 

 object suspended in the air. 



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

 does not cause any noticeable interference with sight, and is not even 

 observed by the individual ; since the visual impression of external 

 objects appears to be continuous during the short interval occupied by 

 the movement of the lids. 



The local sensibility of the retina is diminished by continued visual 

 impressions. This diminution of the retinal sensibility appears to be 

 continuous from the very commencement of a visual impression, so that 

 it may be made perceptible within a few seconds. In the experiment of 

 exhibiting the image of the retinal bloodvessels by changing the posi- 

 tion of their shadows (page 622) these shadows are visible for an instant 

 with extreme sharpness. But they begin to fade almost at once and 

 after a short interval become imperceptible. They can only be seen for 

 a considerable time, by keeping the light in motion, so that the shadows 

 fall alternately upon different parts of the retina. The portions of the 

 retina which are in full illumination have their sensibility so rapidly 

 diminished, that the shadow, if motionless, is no longer visible by con- 

 trast. Those which are in shadow, on the other hand, become compara- 

 tively more sensitive by repose ; and when the shifting of the light 

 brings them again into illumination, they not only receive more stimulus 

 than the adjacent parts, but are also more impressible to its influence. 



If one eye be covered by a dark glass, and the other be used ex- 

 clusively, for an hour or two, in reading or writing, at the end of that 

 time the difference in retinal sensibility of the two eyes will be very 

 apparent. A single faintly luminous object in a dark room may then 

 be almost imperceptible to the eye which has been in use, while it will 

 appear to the other quite brilliant. If the application of the eye have 

 not been carried beyond the bounds of moderation, this difference is 

 transitory ; and by reversing the conditions, that is, covering the eye 

 previously in use, and reading or writing by aid of the other, that which 

 was before the most sensitive to light becomes less so, and that which 

 was previously fatigued recovers its sensibility. 



The alternate diminution and recovery of the retinal sensibility, by 

 excitement and repose, is directly connected with the phenomena of 

 negative images. If the eye be steadily fixed for a short time upon a 



