Though we have thus seen reason to deny to Dr Reid the merit commonly ascribed to him, on the points which we have been considering, relative to the theory of perception, I trust you will not on that account, be insensible to the merits which he truly possessed. He knows little, indeed, of the human mind, who does not know, how compatible many errors and misconceptions are with the brightest and most active energies of intellect. On this “Isthmus of a middle State,” of which Pope speaks, man, though not “reasoning but to err,” is yet subject to occasional error, even in his proudest reasonings. With all his wisdom, he is still but “darkly wise;” and with all the grandeur of his being, but “rudely great.”
VISION.
Our inquiry into the nature of the sensations of touch,—or at least of those sensations, which are truly, and of others which are commonly, though, I think, falsely, ascribed to this organ, has led us into speculations, in the course of which I have been obliged to anticipate many remarks, that more peculiarly belong to the sense which still remains to be considered by us,—the sense of sight, that to which we owe so much of our most valuable information, with respect to nature, and so many of those pleasures, which the bounty of Him, who, has formed us to be happy, as well as to be wise, has so graciously intermingled, with all the primary means of our instruction.
The anticipations, into which I have been led, were necessary for throwing light on the subjects before considered, particularly on the complex feelings ascribed to touch,—the knowledge of which feelings, however, was still more necessary, for understanding fully the complex perceptions of this sense. It is thus scarcely possible, in science, to treat of one subject, without considering it in relation to some other subject, and often to subjects between which, on first view, it would be difficult to trace any relation. Every thing throws light upon every thing,—though the reflection,—which is, in many cases, so bright, as to force itself upon common eyes—may, in other cases, be so faint, as to be perceptible only to eyes of the nicest discernment. It may almost be said, that there is an universal affinity in truths,—like that universal attraction, which unites to each other, as one common system, the whole masses which are scattered through the infinity of space, and by which, as I have before remarked, the annihilation of a single particle of matter, in any one of these orbs,—however inconceivably slight its elementary modification might be of the general sum of attraction,—would in that very instant be productive of change throughout the universe. It is not easy to say, what any one science would have been, if any other science had not existed. How different did Astronomy become, in consequence of the accidental burning of a few sea-weeds upon the sand, to which the origin of glass has been ascribed; and, when we think of the universal accessions, which navigation has made to every department of knowledge, what an infinity of truths may be considered as almost starting into existence, at the moment, when the polarity of the magnet was first observed!
“True to the pole, by thee the pilot guides
His steady helm, amid the struggling tides,
Braves with broad sail the unmeasurable sea,
Cleaves the dark air, and asks no star but thee.”[115]
The anticipations, which have been made, in the present instance, will be of advantage, in abridging much of the labour, which would have been necessary in treating of vision simply. I may now safely leave you, to make, for yourselves, the application of many arguments, on which I have dwelt at length, in treating of the other senses.
The organ of sight, as you well know, is the eye,—a machine of such exquisite and obvious adaptation to the effects produced by it, as to be, of itself, in demonstrating the existence of the Divine Being who contrived it, equal in force to many volumes of theology. The atheist, who has seen, and studied, its internal structure, and yet continues an atheist, may be fairly considered as beyond the power of mere argument to reclaim. The minute details of its structure, however, belong to the anatomist. It is enough for our purpose to know, that, by an apparatus of great simplicity, all the light, which, from every quarter strikes on the pellucid part of the ball of the eye,—and which if it continued to pass in the same direction, would thus produce one mingled and indistinct expanse of colour,—is so refracted, as it is termed, or bent from its former direction to certain focal points, as to be distributed again on the retina, in distinct portions, agreeing with the portions which come from each separate object, so exactly, as to form on it a miniature landscape of the scenery without. Nor is this all. That we may vary, at our pleasure, the field of this landscape, the ball of the eye is furnished with certain muscles, which enable us to direct it more particularly toward the objects which we wish to view; and according as the light which falls from these may be more or less intense, there are parts which minister to the sensibility of the eye, by increasing or diminishing in proportion the transparent aperture at which the light is admitted. There are, then, in this truly wonderful and beautiful process, in the first place, as determining what objects, in the wide scene around us, are to be visible at the moment, the contraction of certain muscles, on which the particular field of our vision depends, and which may almost be said to enable us to increase the extent of our field of vision, by enabling us to vary it at will;—in the second place, the external light, emitted from all the objects within this radiant field, which, on its arrival at the retina, is itself the direct object of vision; in the third place, the provision for increasing or diminishing the diameter of the pupil, in proportion to the quantity of that incident light;—in the fourth place, the apparatus, by which the dispersed rays of light are made to assume within the eye, the focal convergence necessary for distinct vision;—and lastly, the expansion of the optic nerve, as a part of the great sensorial organ, essential to sensation. The difference of the phenomena, produced by the varieties of the external light itself, is exhibited in almost every moment of our waking existence; and the diversities, arising from other parts of the process, are not less striking. There are peculiar diseases which affect the optic nerve, or other parts of the sensorial organ immediately connected with it,—there are other diseases which affect the refracting apparatus,—others which affect the iris, so as to prevent the enlargement or diminution of the pupil, when different quantities of light are poured on it,—others, which affect the muscles that vary the position of the ball,—and, in all these cases, we find, as might be expected, a corresponding difference of the phenomena.