the trains which present themselves in madness, phrensies, and common deliriums. This agrees remarkably with what has been already observed concerning the ideas of this sense; and we may infer from all together, that the regions of the brain corresponding to the optic nerve are comparatively large, or peculiarly susceptive of impressions, or both.
Tenthly, The imagery of the eye sympathizes also remarkably with the affections of the stomach. Thus the grateful impressions of opium upon the stomach raise up the ideas of gay colours, and transporting scenes, in the eye; and spasms, and indigestions, have often a contrary effect. The ghastly faces which sometimes appear in idea, particularly after drinking tea, seem to be an effect of this kind, or perhaps of the last-mentioned one; for they are common to persons of irritable nervous systems. Ghastly faces may take place preferably to other disagreeable ideas, perhaps because characters, affections, passions, are principally denoted and expressed by the countenance; because faces are the most common of visible objects, and attended to with the greatest earnestness; because we criticize much upon the beauty of faces, and upon the proportion of the several features to each other; and because evil spirits (the notions of which generally take strong and early possession of our fancies) are painted with ghastly faces. This mixture of reasons hinders each particular one from being so obvious, as might otherwise be expected; however, the same thing is common in many other cases. The trains of visible ideas, which occur in dreams, are deducible, partly from the sympathy here mentioned, partly from that of the last paragraph.
Eleventhly, Our stock of visible ideas may be considered as a key to a great part in our knowledge, and a principal source of invention in poetry, painting, mathematics, mechanics, and almost every other branch of the arts and sciences. In mathematics and mechanics the invention of the diagram is, in effect, the solution of the problem. Our memories are also much assisted by our visible ideas in respect of past facts, and the preservation of the order of time depends in a particular manner upon our visible trains suggesting each other in due succession. Hence eye-witnesses generally relate in order of time, without any express design of doing so. This recollection of visible ideas, in the order in which they were impressed, gives rise to the loci memoriales, in which matters principally worthy of remembrance are to be reposited, and to the artificial memory, that is borrowed from the eye; just as the facility of remembering words formed into verses does to the artificial memory borrowed from the ear. It may deserve notice here, that some persons have imaginary places for the natural numbers, as far as 100, or farther.
Twelfthly, The ideas which different persons have of the same persons’ faces, though they be very like one another, cannot yet be precisely the same, on account of the addition and omission
