s some of
its powers from being mixed and surrounded with colored bodies. In its
own nature, it cannot be considered as a color. Black bodies, reflecting
none, or but a few rays, with regard to sight, are but as so many vacant
spaces, dispersed among the objects we view. When the eye lights on one
of these vacuities, after having been kept in some degree of tension by
the play of the adjacent colors upon it, it suddenly falls into a
relaxation; out of which it as suddenly recovers by a convulsive spring.
To illustrate this: let us consider that when we intend to sit on a
chair, and find it much lower than was expected, the shock is very
violent; much more violent than could be thought from so slight a fall
as the difference between one chair and another can possibly make. If,
after descending a flight of stairs, we attempt inadvertently to take
another step in the manner of the former ones, the shock is extremely
rude and disagreeable: and by no art can we cause such a shock by the
same means when we expect and prepare for it. When I say that this is
owing to having the change made contrary to expectation; I do not mean
solely, when the _mind_ expects. I mean likewise, that when any organ of
sense is for some time affected in some one manner, if it be suddenly
affected otherwise, there ensues a convulsive motion; such a convulsion
as is caused when anything happens against the expectance of the mind.
And though it may appear strange that such a change as produces a
relaxation should immediately produce a sudden convulsion; it is yet
most certainly so, and so in all the senses. Every one knows that sleep
is a relaxation; and that silence, where nothing keeps the organs of
hearing in action, is in general fittest to bring on this relaxation;
yet when a sort of murmuring sounds dispose a man to sleep, let these
sounds cease suddenly, and the person immediately awakes; that is, the
parts are braced up suddenly, and he awakes. This I have often
experienced myself, and I have heard the same from observing persons. In
like manner, if a person in broad daylight were falling asleep, to
introduce a sudden darkness would prevent his sleep for that time,
though silence and darkness in themselves, and not suddenly introduced,
are very favorable to it. This I knew only by conjecture on the analogy
of the senses when I first digested these observations; but I have since
experienced it. And I have often experienced, and so have a thousan