VISUAL PERCEPTIONS. 369 



When the impressions are very vivid, another phenome- 

 non often takes place; namely, their suhsequent recurrence, 

 after a certain interval, during whicli tliey are not felt, and 

 quite independently of any renewed aj)plicatioii of the cause 

 which had originally excited them. If, for example, we 

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

 mediately 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; hut if we continue to keep the eyes shut, the 

 same impression 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 pro- 

 bable that these reappearances of the image, after the light 

 which produced the original impressioji has been withdrawn, 

 are occasioned by spontaneous affections of the retina itself, 

 which are conveyed to the sensorium. In other cases, 

 where the impressions are less strong, the physical changes 

 producing these spectra are perhaps confined to the senso- 

 rium. These spectral appearances generally undergo vari- 

 ous 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 impressions 

 made on the nerves of sense tend to exliaust their sensibility, 

 so that the continued or renewed action of the same exter- 

 nal cause produces a less effect than at first: while, on tiie 

 other hand, the absence or diminution of the usual excite- 

 ment leads to a gradual increase of sensibility, so that the 

 subsequent application of an exciting cause produces more 

 than the usual effect. One of the most obvious exemplifica- 

 tions of this law presents itself in tlic case of the sensations 

 of temperature. The very same body may appear warm to 



mascope or Pheniiklsticope. I constructed several of these at that period, 

 (in the spring- of 1 831 ) wiiich I showed to many of my friends; but in conse- 

 quence of occupations and cares of a more serious kind, I did not publish any 

 account of this invention, which was last year reproduced on Uic continent. 



Vol. II. 47 



