224 Dr. Darwin’s Experiments, on- the 
IV. OF DIRECT OCULAR SPECTRA# 
A quantity of fimulus fomewhat greater than natural excites 
the retina into fpafmodlc action, which ceafes in a few feconds . 
A certain duration and energy of the ftimulus of light and 
colours excites the perfect adtion of the retina in vifion ; for 
tery quick motions are imperceptible to us, as well as very 
flow ones, as the whirling of a top, or the fhadow on a fun- 
dial. So perfeft darknefs does not affedt the eye at all ; and 
excefs of light produces pain, not vifion. 
j 9 When a fire-coal is whirled round in the dark, a lucid 
circle remains a considerable time in the eye \ and that with fo 
much vivacity of light, that it is miftaken for a continuance of 
the irritation of the object. In the fame manner, when a. 
fiery meteor (hoots acrofs the night, it appears to leave a long^ 
lucid train behind it, part of which,- and perhaps Sometimes 
the whole, is owing to the continuance of the adtionof the retina 
after having been thus vividly excited. This is beautifully 
illuftrated by the following experiment : fix a paper fail, three 
or four inches in diameter, and made like that of a fmoke jack, 
in a tube of pafteboard; on looking through the tube at a 
diftant profpedt, fome disjointed parts of it will be feen 
through the narrow intervals between the fails ; but as the fly 
begins to revolve, thefe intervals appear larger ; and when it 
revolves quicker, the whole profpeft is feen quite as diftinft; 
as if nothing intervened, though lefs luminous. 
2. Look through a dark tube, about half a yard long, at~ 
the area of a yellow circle of half an inch diameter, lying 
upon a blue area of double that diameter, for half a minute; 
i and 
