action of vomiting in like manner ceases, and is renewed by intervals,
although the emetic drug is thrown up with the first effort: so after-pains
continue some time after parturition; and the alternate pulsations of the
heart of a viper are renewed for some time after it is cleared from its
blood.

VI. OF REVERSE OCULAR SPECTRA.

_The retina, after having been excited into action by a stimulus
somewhat greater them the last mentioned, falls into opposite spasmodic
action._

The actions of every part of animal bodies may be advantageously compared
with each other. This strict analogy contributes much to the investigation
of truth; while those looser analogies, which compare the phenomena of
animal life with those of chemistry or mechanics, only serve to mislead our
inquiries.

When any of our larger muscles have been in long or in violent action, and
their antagonists have been at the same time extended, as soon as the
action of the former ceases, the limb is stretched the contrary way for our
ease, and a pandiculation or yawning takes place.

By the following observations it appears, that a similar circumstance
obtains in the organ of vision; after it has been fatigued by one kind of
action, it spontaneously falls into the opposite kind.

1. Place a piece of coloured silk, about an inch in diameter, on a sheet of
white paper, about half a yard from your eyes; look steadily upon it for a
minute; then remove your eyes upon another part of the white paper, and a
spectrum will be seen of the form of the silk thus inspected, but of a
colour opposite to it. A spectrum nearly similar will appear if the eyes
are closed, and the eyelids shaded by approaching the hand near them, so as
to permit some, but to prevent too much light falling on them.

Red silk produced a green spectrum.
Green produced a red one.
Orange produced blue.
Blue produced orange.
Yellow produced violet.
Violet produced yellow.

That in these experiments the colours of the spectra are the reverse of the
colours which occasioned them, may be seen by examining the third figure in
Sir Isaac Newton's Optics, L. II. p. 1, where those thin laminae of air,
which reflected yellow, transmitted violet; those which reflected red,
transmitted a blue green; and so of the rest, agreeing with the experiments
above related.

2. These reverse spectra are similar to a colour, formed by a combination
of all the primary colours except that