y of mind, or even
too great attention to the spectrum itself, will disappoint them, till they
have had a little experience in attending to such small sensations.

The spectra described in this section, termed direct ocular spectra, are
produced without much fatigue of the eye; the irritation of the luminous
object being soon withdrawn, or its quantity of light being not so great as
to produce any degree of uneasiness in the organ of vision; which
distinguishes them from the next class of ocular spectra, which are the
consequence of fatigue. These direct spectra are best observed in such
circumstances that no light, but what comes from the object, can fall upon
the eye; as in looking through a tube, of half a yard long, and an inch
wide, at a yellow paper on the side of a room, the direct spectrum was
easily produced on closing the eye without taking it from the tube; but if
the lateral light is admitted through the eyelids, or by throwing the
spectrum on white paper, it becomes a reverse spectrum, as will be
explained below.

The other senses also retain for a time the impressions that have been made
upon them, or the actions they have been excited into. So if a hard body is
pressed upon the palm of the hand, as is practised in tricks of
legerdemain, it is not easy to distinguish for a few seconds whether it
remains or is removed; and tastes continue long to exist vividly in the
mouth, as the smoke of tobacco, or the taste of gentian, after the sapid
material is withdrawn.

V. _A quantity of stimulus somewhat greater than the last mentioned
excites the retina into spasmodic action, which ceases and recurs
alternately._

1. 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. This yellow spectrum of the sun when the
eyelids are opened becomes blue; and if it is made to fall on the green
grass, or on other coloured objects, it varies its own colour by an
intermixture of theirs, as will be explained in another place.

2. Place a lighted spermaceti candle in the night about one foot from your
eye, and look steadily on the centre of the flame, till your eye becomes
much more fatigued than in Sect. IV. Exp. 3.; and on closing your eyes a
reddish spectrum will be perceived, which will cease and return
alternately.

The