The effects of blackness.

Blackness is but a partial darkness; and therefore it derives 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 ............