ight it
self produces the sensation of a Colour, but as it produces such a
determinate kind of local motion in some part of the brain; which, though
it happen most commonly from the motion whereinto the slender strings of
the _Retina_ are put, by the appulse of Light, yet if the like motion
happen to be produc'd by any other cause, wherein the Light concurrs not at
all, a man shall think he sees the same Colour. For proof of this, I might
put you in mind, that 'tis usual for dreaming men to think they see the
Images that appear to them in their sleep, adorn'd some with this, and some
with that lively Colour, whilst yet, both the curtains of their bed, and
those of their eyes are close drawn. And I might add the confidence with
which distracted persons do oftentimes, when they are awake, think, they
see black fiends in places, where there is no black object in sight without
them. But I will rather observe, that not only when a man receives a great
stroak upon his eye, or a very great one upon some other part of his head,
he is wont to see, as it were, flashes of lightning, and little vivid, but
vanishing flames, though perhaps his eyes be shut: But the like apparitions
may happen, when the motion proceeds not from something without, but from
something within the body, provided the unwonted fumes that wander up and
down in the head, or the propagated concussion of any internal part in the
body, do cause about the inward extremities of the Optick Nerve, such a
motion as is wont to be there produc'd, when the stroak of the Light upon
the _Retina_ makes us conclude, that we see either Light, or such and such
a Colour: This the most ingenious _Des Cartes_ hath very well observ'd, but
because he seems not to have exemplifi'd it by any unobvious or peculiar
observation, I shall indeavour to illustrate this doctrine by a few
Instances.

5 And first, I remember, that having, through Gods goodness, been free for
several years, from troublesome Coughs, being afterwards, by an accident,
suddenly cast into a violent one, I did often, when I was awaked in the
night by my distempers, observe, that upon coughing strongly, it would seem
to mee, that I saw very vivid, but immediately disappearing flames, which I
took particular notice of, because of the conjecture I am now mentioning.

6 An excellent and very discreet person, very near ally'd both to you and
mee, was relating to mee, that some time since, whilst she was talking with
some