t star, or of a distant candle in the night, is
perhaps owing to the same cause. While we continue to look upon these
luminous objects, their central parts gradually appear paler, owing to the
decreasing sensibility of the part of the retina exposed to their light;
whilst, at the same time, by the unsteadiness of the eye, the edges of them
are perpetually falling on parts of the retina that were just before
exposed to the darkness of the night, and therefore tenfold more sensible
to light than the part on which the star or candle had been for some time
delineated. This pains the eye in a similar manner as when we come suddenly
from a dark room into bright daylight, and gives the appearance of bright
scintillations. Hence the stars twinkle most when the night is darkest, and
do not twinkle through telescopes, as observed by Musschenbroeck; and it
will afterwards be seen why this twinkling is sometimes of different
colours when the object is very bright, as Mr. Melvill observed in looking
at Sirius. For the opinions of others on this subject, see Dr. Priestley's
valuable History of Light and Colours, p. 494.

Many facts observable in the animal system are similar to these; as the hot
glow occasioned by the usual warmth of the air, or our clothes, on coming
out of a cold bath; the pain of the fingers on approaching the fire after
having handled snow; and the inflamed heels from walking in snow. Hence
those who have been exposed to much cold have died on being brought to a
fire, or their limbs have become so much inflamed as to mortify. Hence much
food or wine given suddenly to those who have almost perished by hunger has
destroyed them; for all the organs of the famished body are now become so
much more irritable to the stimulus of food and wine, which they have long
been deprived of, that inflammation is excited, which terminates in
gangrene or fever.

IV. OF DIRECT OCULAR SPECTRA.

_A quantity of stimulus somewhat greater than natural excites the
retina into spasmodic action, which ceases in a few seconds._

A certain duration and energy of the stimulus of light and colours excites
the perfect action of the retina in vision; for very quick motions are
imperceptible to us, as well as very slow ones, as the whirling of a top,
or the shadow on a sun-dial. So perfect darkness does not affect the eye at
all; and excess of light produces pain, not vision.

1. When a fire-coal is whirled round in the dark, a lucid ci