us, in the inirritative or nervous fever, the pupil of the eye becomes
dilated; which in this, as well as in the dropsy of the brain, is generally
a fatal symptom. A part of the cornea as well as a part of the albuginea in
these fevers is frequently seen during sleep; which is owing to the
inirritability of the retina to light, or to the general paresis of
muscular action, and in consequence to the less contraction of the
sphincter of the eye, if it may be so called, at that time.

There have been instances of some, who could not distinguish certain
colours; and yet whose eyes, in other respects, were not imperfect. Philos.
Transact. Which seems to have been owing to the want of irritability, or
the inaptitude to action, of some classes of fibres which compose the
retina. Other permanent defects depend on the diseased state of the
external organ. Class I. 1. 3. 14. I. 2. 3. 25. IV. 2. 1. 11.

3. _Muscae volitantes._ Dark spots appearing before the eyes, and changing
their apparent place with the motions of the eyes, are owing to a temporary
defect of irritability of those parts of the retina, which have been lately
exposed to more luminous objects than the other parts of it, as explained
in Sect. XL. 2. Hence dark spots are seen on the bed-clothes by patients,
when the optic nerve is become less irritable, as in fevers with great
debility; and the patients are perpetually trying to pick them off with
their fingers to discover what they are; for these parts of the retina of
weak people are sooner exhausted by the stimulus of bright colours, and are
longer in regaining their irritability.

Other kinds of ocular spectra, as the coloured ones, are also more liable
to remain in the eyes of people debilitated by fevers, and to produce
various hallucinations of sight. For after the contraction of a muscle, the
fibres of it continue in the last situation, till some antagonist muscles
are exerted to retract them; whence, when any one is much exhausted by
exercise, or by want of sleep, or in fevers, it is easier to let the fibres
of the retina remain in their last situation, after having been stimulated
into contraction, than to exert any antagonist fibres to replace them.

As the optic nerves at their entrance into the eyes are each of them as
thick as a crow-quill, it appears that a great quantity of sensorial power
is expended during the day in the perpetual activity of our sense of
vision, besides that used in the motions of