SECT. XVII.—THE EFFECTS OF BLACKNESS
BLACKNESS is but a partial darkness; and therefore, it derives some of its powers from being mixed and surrounded with coloured bodies. In its own nature, it cannot be considered as a colour. 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 colours 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 day-light 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 favourable 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 thousand others, that on the first inclining towards sleep, we have been suddenly awakened with a most violent start; and that this start was generally preceded by a sort of dream of our falling down a precipice: whence does this strange motion arise, but from the too sudden relaxation of the body, which by some mechanism in nature restores itself by as quick and vigorous an exertion of the contracting power of the muscles? The dream itself is caused by this relaxation; and it is of too uniform a nature to be attributed to any other cause. The parts relax too suddenly, which is in the nature of falling; and this accident of the body induces this image in the mind. When we are in a confirmed state of health and vigour, as all changes are then less sudden, and less on the extreme, we can seldom complain of this disagreeable sensation.