her bodies to which the spectrum is supposed to be
contiguous, but to other causes mentioned in No. X. 3. of this section. The
apparent size of it will also be variable according to its supposed
distance.

As these spectra are more easily observable when our eyes are a little
weakened by fatigue, it has frequently happened, that people of delicate
constitutions have been much alarmed at them, fearing a beginning decay of
their sight, and have thence fallen into the hands of ignorant oculists;
but I believe they never are a prelude to any other disease of the eye, and
that it is from habit alone, and our want of attention to them, that we do
not see them on all objects every hour of our lives. But as the nerves of
very weak people lose their sensibility, in the same manner as their
muscles lose their activity, by a small time of exertion, it frequently
happens, that sick people in the extreme debility of fevers are perpetually
employed in picking something from the bed-clothes, occasioned by their
mistaking the appearance of these _muscae volitantes_ in their eyes.
Benvenuto Celini, an Italian artist, a man of strong abilities, relates,
that having passed the whole night on a distant mountain with some
companions and a conjurer, and performed many ceremonies to raise the
devil, on their return in the morning to Rome, and looking up when the sun
began to rise, they saw numerous devils run on the tops of the houses, as
they passed along; so much were the spectra of their weakened eyes
magnified by fear, and made subservient to the purposes of fraud or
superstition. (Life of Ben. Celini.)

3. Place a square inch of white paper on a large piece of straw-coloured
silk; look steadily some time on the white paper, and then move the centre
of your eyes on the silk, and a spectrum of the form of the paper will
appear on the silk, of a deeper yellow than the other part of it: for the
central part of the retina, having been some time exposed to the stimulus
of a greater quantity of white light, is become less sensible to a smaller
quantity of it, and therefore sees only the yellow rays in that part of the
straw-coloured silk.

Facts similar to these are observable in other parts of our system: thus,
if one hand be made warm, and the other exposed to the cold, and then both
of them immersed in subtepid water, the water is perceived warm to one
hand, and cold to the other; and we are not able to hear weak sounds for
some time after