In our instance, there was no time for such a habit; and there is no reason to think that the ill effects of black on his imagination were more owing to its connection with any disagreeable ideas, than that the good effects of more cheerful colors were derived from their connection with pleasing ones.

They had both probably their effects from their natural operation.
SECTION XVI.
WHY DARKNESS IS TERRIBLE.
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 proportionally 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.