VISUAL PERCEPTIONS. 525 



during which they are not felt, and quite in- 

 dependently 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 con- 

 tinue to keep the eyes shut, the same impression 

 will, after a certain time, recur, and again vanish ; 

 and this phenomenon will be repeated at inter- 

 vals, the sensation becoming fainter at each re- 

 newal. It is probable that these reappearances 

 of the image, after the light which produced the 

 original impression has been withdrawn, are oc- 

 casioned 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 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 

 impressions made on the nerves of sense tend to 

 exhaust their sensibility, so that the continued or 

 renewed action of the same external cause pro- 

 duces a less effect than at first : while, on the 



