344 iT\ Darwin’s Experiments on the 
in the appearance of the fpe&rum, not only in Its vivacity, but 
in its colour ; as the direct fpeftrum of the central object or of 
the circumjacent ones, and alfo the reverfe fpeclra of both, 
with their various combinations, as well as the time of their 
duration in the eye, and of their remifiions or alternations, 
depend upon the degree of fatigue the retina is fubje&ed to. 
The Chevalier d’Arcv conftrudked a machine by which a coal 
of fire was whirled round in the dark, and found, that when a 
luminous body made a revolution in eight thirds of time, it 
prefented to the eye a complete circle of fire ; from whence he 
concludes, that the impreffion continues on the organ about 
the feventh part of a fecond. (Mem. de I’Acad. des Sc. 
1765.) This, however, is only to be confidered as the fhorteft 
time of the duration of thefe diredt fpedtra ; fince in the fa- 
tigued eye both the direct and reverfe fpe&ra, with their 
intermiffions, appear to take up many feconds of time, and 
feem very variable in proportion to the circumftances of 
fatigue or energy. 
4. It fometimes happens, if the eyeballs have been rubbed 
hard with the fingers, that lucid fparks are feen in quick mo- 
tion amidft the fpe&rum we are attending to. This is fimilar 
to the fiafhes of fire from a ftroke on the eye in fighting,' and 
is refembled by the warmth and glow which appear upon the 
fkin after friction, and is probably owing to an acceleration of the 
arterial blood into the veffels emptied by the previous preffure. 
By being accuftomed to obferve fuch fmall fenfations in the eye, 
it is eafy to fee the circulation of the blood in this organ. I 
have attended to this frequently, when I have obferved my 
eyes more than commonly fenfible to other fpeftra. The 
circulation may be feen either in both eyes at a time, or only 
in one of them ; for as a certain quantity of light is neceflary 
* to 
