RECURRENCE OF PERCEPTIOXS. 467 



When the impressions are very vivid, another 

 phenomenon often takes phice ; namely, their sub- 

 sequent recurrence, after a certain interval, during 

 which they are not felt, and quite independently of 

 any renewed application of the cause which had 

 originally excited them. If, for example, we look 

 steadfastly at the sun for a second or two, and then 

 immediately close our eyes, the image, or spectrum 

 of the sun remains for a long time present to the 

 mind, as if its light were still acting on the retina. 

 It then gradually fades and disappears ; but if we 

 continue to keep the eyes shut, the same impres- 

 sion will, after a certain time, recur, and again 

 vanish ; and this phenomenon will be repeated at 

 intervals, the sensation becoming fainter at each 

 renewal. It is probable that these reappearances 

 of the image, after the light which produced the 

 original impression has been withdrawn, are occa- 

 sioned by spontaneous affections of the retina itself, 

 w^hich are conveyed to the sensorium. In other 

 cases, where the impressions are less strong, the 

 physical changes producing these spectra are per- 

 haps confined to the sensorium. These spectral 

 appearances generally undergo various changes of 

 colour; assuming first a yellow tint; passing then 

 to a green ; and lastly becoming blue, before they 

 finally disappear. 



Another general law of sensation is, that all im- 

 pressions made on the nerves of sense tend to ex- 

 haust their sensibility; so that the continued or 

 renewed action of the same external cause produces 

 a less effect than at first ; while, on the other hand, 

 the absence or diminution of the usual excitement 

 leads to a gradual increase of sensibility, so that the 



