ns of spectra bear some
analogy to the tremors of the hands, and palpitations of the heart, of weak
people: and perhaps a criterion of the strength of any muscle or nerve may
be taken from the time it can be continued in exertion.

4. _Variation of spectra in respect to brilliancy; the visibility of the
circulation of the blood in the eye._

1. The meridian or evening light makes a difference in the colours of some
spectra; for as the sun descends, the red rays, which are less refrangible
by the convex atmosphere, abound in great quantity. Whence the spectrum of
the light parts of a window at this time, or early in the morning, is red;
and becomes blue either a little later or earlier; and white in the
meridian day; and is also variable from the colour of the clouds or sky
which are opposed to the window.

2. All these experiments are liable to be confounded, if they are made too
soon after each other, as the remaining spectrum will mix with the new
ones. This is a very troublesome circumstance to painters, who are obliged
to look long upon the same colour; and in particular to those whose eyes,
from natural debility, cannot long, continue the same kind of exertion. For
the same reason, in making these experiments, the result becomes much
varied if the eyes, after viewing any object, are removed on other objects
for but an instant of time, before we close them to view the spectrum; for
the light from the object, of which we had only a transient view, in the
very time of closing our eyes acts as a stimulus on the fatigued retina;
and for a time prevents the defined spectrum from appearing, or mixes its
own spectrum with it. Whence, after the eyelids are closed, either a dark
field, or some unexpected colours, are beheld for a few seconds, before the
desired spectrum becomes distinctly visible.

3. The length of time taken up in viewing an object, of which we are to
observe the spectrum, makes a great difference in the appearance of the
spectrum, not only in its vivacity, but in its colour; as the direct
spectrum of the central object, or of the circumjacent ones, and also the
reverse spectra of both, with their various combinations, as well as the
time of their duration in the eye, and of their remissions or alternations,
depend upon the degree of fatigue the retina is subjected to. The Chevalier
d'Arcy constructed a machine by which a coal of fire was whirled round in
the dark, and found, that when a luminous body mad