rcle remains a
considerable time in the eye; and that with so much vivacity of light, that
it is mistaken for a continuance of the irritation of the object. In the
same manner, when a fiery meteor shoots across 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 action of the retina after
having been thus vividly excited. This is beautifully illustrated by the
following experiment: fix a paper sail, three or four inches in diameter,
and made like that of a smoke jack, on a tube of pasteboard; on looking
through the tube at a distant prospect, some disjointed parts of it will be
seen through the narrow intervals between the sails; but as the fly begins
to revolve, these intervals appear larger; and when it revolves quicker,
the whole prospect is seen quite as distinct as if nothing intervened,
though less luminous.

[Illustration: Fig. 3.]

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; and on closing your eyes the colours of
the spectrum will appear similar to the two areas, as in fig. 3.; but if
the eye is kept too long upon them, the colours of the spectrum will be the
reverse of those upon the paper, that is, the internal circle will become
blue, and the external area yellow; hence some attention is required in
making this experiment.

3. Place the bright flame of a spermaceti candle before a black object in
the night; look steadily at it for a short time, till it is observed to
become somewhat paler; and on closing the eyes, and covering them
carefully, but not so as to compress them, the image of the blazing candle
will continue distinctly to be visible.

4. Look steadily, for a short time, at a window in a dark day, as in Exp.
2. Sect. III. and then closing your eyes, and covering them with your
hands, an exact delineation of the window remains for some time visible in
the eye. This experiment requires a little practice to make it succeed
well; since, if the eyes are fatigued by looking too long on the window, or
the day be too bright, the luminous parts of the window will appear dark in
the spectrum, and the dark parts of the frame-work will appear luminous, as
in Exp. 2. Sect. III. And it is even difficult for many, who first try this
experiment, to perceive the spectrum at all; for any hurr