we have been exposed to loud ones; and we feel a chilliness
on coming into an atmosphere of temperate warmth, after having been some
time confined in a very warm room: and hence the stomach, and other organs
of digestion, of those who have been habituated to the greater stimulus of
spirituous liquor, are not excited into their due action by the less
stimulus of common food alone; of which the immediate consequence is
indigestion and hypochondriacism.

III. OF SPECTRA FROM EXCESS OF SENSIBILITY.

_The retina is more easily excited into action by greater irritation
after having been lately subjected to less._

1. If the eyes are closed, and covered perfectly with a hat, for a minute
or two, in a bright day; on removing the hat a red or crimson light is seen
through the eyelids. In this experiment the retina, after being some time
kept in the dark, becomes so sensible to a small quantity of light, as to
perceive distinctly the greater quantity of red rays than of others which
pass through the eyelids. A similar coloured light is seen to pass through
the edges of the fingers, when the open hand is opposed to the flame of a
candle.

2. If you look for some minutes steadily on a window in the beginning of
the evening twilight, or in a dark day, and then move your eyes a little,
so that those parts of the retina, on which the dark frame-work of the
window was delineated, may now fall on the glass part of it, many luminous
lines, representing the frame-work, will appear to lie across the glass
panes: for those parts of the retina, which were before least stimulated by
the dark frame-work, are now more sensible to light than the other parts of
the retina which were exposed to the more luminous parts of the window,

3. Make with ink on white paper a very black spot, about half an inch in
diameter, with a tail about an inch in length, so as to represent a
tadpole, as in plate 2, at Sect. III. 3. 3.; look steadily for a minute on
this spot, and, on moving the eye a little, the figure of the tadpole will
be seen on the white part of the paper, which figure of the tadpole will
appear whiter or more luminous than the other parts of the white paper; for
the part of the retina on which the tadpole was delineated, is now more
sensible to light, than the other parts of it, which were exposed to the
white paper. This experiment is mentioned by Dr. Irwin, but is not by him
ascribed to the true cause, namely, the greater sensibil