On the Sublime and Beautiful/Part IV/Chapter 16
IT may be worth while to examine how darkness can operate in such a manner as to cause pain. It is observable, that still as we recede from the light, nature has so contrived it, that the pupil is enlarged by the retiring of the iris, in proportion to our recess. Now, instead of declining from it but a little, suppose that we withdraw entirely from the light; it is reasonable to think, that the contraction of the radial fibres of the iris is proportionably greater; and that this part may by great darkness come to be so contracted as to strain the nerves that compose it beyond their natural tone; and by this means to produce a painful sensation. Such a tension it seems there certainly is, whilst we are involved in darkness; for in such a state, whilst the eye remains open, there is a continual nisus to receive light; this is manifest from the flashes and luminous appearances which often seem in these circumstances to play before it; and which can be nothing but the effect of spasms, produced by its own efforts in pursuit of its object: several other strong impulses will produce the idea of light in the eye, besides the substance of light itself, as we experience on many occasions. Some, who allow darkness to be a cause of the sublime, would infer, from the dilatation of the pupil, that a relaxation may be productive of the sublime, as well as a convulsion: but they do not, I believe, consider that although 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 apprized 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.