the circular ring of the iris be in some sense a sphincter, which may possibly be dilated by a simple relaxation, yet in one respect it differs from most of the other sphincters of the body, that it is furnished with antagonist muscles, which are the radial fibres of the iris:  no sooner does the circular muscle begin to relax, than these fibres, wanting their counterpoise, are forcibly drawn back, and open the pupil to a considerable wideness.  But though we were not apprised of this, I believe any one will find, if he opens his eyes and makes an effort to see in a dark place, that a very perceivable pain ensues.  And I have heard some ladies remark, that after having worked a long time upon a ground of black, their eyes were so pained and weakened, they could hardly see.  It may perhaps be objected to this theory of the mechanical effect of darkness, that the ill effects of darkness or blackness seem rather mental than corporeal:  and I own it is true that they do so; and so do all those that depend on the affections of the finer parts of our system.  The ill effects of bad weather appear often no otherwise than in a melancholy and dejection of spirits; though without doubt, in this case, the bodily organs suffer first, and the mind through these organs.

SECTION 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 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;