the contractions of animal
fibres being not so easily excited by a less stimulus after the organ has
been subjected to a greater. Thus after looking at any luminous object of a
small size, as at the setting sun, for a short time, so as not much to
fatigue the eye, this part of the retina becomes less sensible to smaller
quantities of light; hence when the eyes are turned on other less luminous
parts of the sky, a dark spot is seen resembling the shape of the sun, or
other luminous object which we last behold. See Sect. XL. No. 2.

Thus we are some time before we can distinguish objects in an obscure room
after coming from bright day-light, though the iris presently contracts
itself. We are not able to hear weak sounds after loud ones. And the
stomachs of those who have been much habituated to the stronger stimulus of
fermented or spirituous liquors, are not excited into due action by weaker
ones.

3. A quantity of stimulus something greater than the last mentioned, or
longer continued, induces the organ into spasmodic action, which ceases and
recurs alternately. Thus on looking for a time on the setting sun, so as
not greatly to fatigue the sight, a yellow spectrum is seen when the eyes
are closed and covered, which continues for a time, and then disappears and
recurs repeatedly before it entirely vanishes. See Sect. XL. No. 5. Thus
the action of vomiting ceases and is renewed by intervals, although the
emetic drug is thrown up with the first effort. A tenesmus continues by
intervals some time after the exclusion of acrid excrement; and the
pulsations of the heart of a viper are said to continue some time after it
is cleared from its blood.

In these cases the violent contractions of the fibres produce pain
according to law 4; and this pain constitutes an additional kind or
quantity of excitement, which again induces the fibres into contraction,
and which painful excitement is again renewed, and again induces
contractions of the fibres with gradually diminishing effect.

4. A quantity of stimulus greater than that last mentioned, or longer
continued, induces the antagonist muscles into spasmodic action. This is
beautifully illustrated by the ocular spectra described in Sect. XL. No. 6.
to which the reader is referred. From those experiments there is reason to
conclude that the fatigued part of the retina throws itself into a contrary
mode of action like oscitation or pandiculation, as soon as the stimulus,
which has