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That the function of the eye and optic nerve is simply to receive and to transmit to the organ of mind, the impressions made on them by the luminous rays with which they are in contact, and that in themselves they do not form any kind of ideas, or judge of any kind of qualities; and that, for this purpose, the co-operation of the brain is absolutely indispensable, will plainly enough appear on an attentive examination of the structure of the eye, and of the phenomena consequent either upon its destruction, or upon its restoration to health.

In the case of vision, as in all other varieties of sensation, the nerve is the really active or essential part, and the ocular apparatus upon which it is distributed differs from that on which other nerves are ramified, from the ear for instance, only because light, or the object to be perceived, differs in its physical properties from the objects cognized by the other nerves, as from the air in the case of hearing. If the nerve of hearing were ramified on the membrances of the eye, it is quite obvious that the non-adaptation of the mechanism of that organ to the propagation of the atmospheric vibrations would render it utterly useless as a nerve of hearing; and if, on the other hand, the nerve of vision were ramified on the internal ear, it is equally obvious that it would be utterly useless, because the rays of light could never reach it. The nerve of vision, then, requires the aid of, and is attached to, the eyeball, solely because the latter is so exquisitely adapted to the properties of light, that while the transparency of its humours allows the free transmission of the luminous rays, the relative convexity and form of its membranes modifies them in their passage, in such a way as to produce a distinct and well-defined image on the nervous expansion, which is destined to receive and transmit the impression to the brain or organ of mind. That this is the real and sole use of the eyeball, and that it does not form ideas of any kind, is ap. parent from the fact, that, even after death, or after extraction from its socket, the eye retains the power of forming a distinct image of external objects on the retina or nerve, so

long as its membranes and humours retain their transparency, and the eye is not suffered to collapse; and also by many instances of disease in which the eyeball remains unaltered, and transmits the luminous rays to the retina with perfect fidelity, but where, from changes in the optic nerve, or in the brain, no cause of sensation is communicated to the mind.

If then it be admitted that the nerve is the essential organ of the external sense, it must also be granted, that, singly, it cannot form ideas or judge of qualities, and that, as in the case of other nerves, it is the mind acting in connexion with the cerebral organs that is their true source. This accordingly is the received phrenological doctrine; but, as much confusion still exists in the public mind on this subject, some farther facts, arguments, and illustrations, may not be misplaced.

If the case were otherwise, and the eye itself were the true source of the kind of ideas already specified, persons born blind, and who consequently could never have enjoyed or manifested any of the functions attached to that organ, ought to possess no notion of form, colour, distance, or magnitude; and, on the principle that destruction of an organ necessarily involves the destruction of the function belonging to it, those who have been so unfortunate as to lose both eyes by accident or by disease, must, if the eye be the real source of these ideas, have lost not simply the power of vision, but also all perception and recollection of the distinguishing qualities of external objects. But when we come to consult nature, precisely the reverse of all this is found to hold good; for not only are ideas of form, colour, and distance, retained in perfect vigour after the loss of both eyes and the total destruction of their function-vision, but many blind people, who have never seen at all, excel in these very perceptions, and have a more accurate conception of the form and properties of a cube or a sphere, than nine-tenths of those who have them always before their eyes. Saunderson, who, it is well known, was born blind, was a capital mathematician and

teacher of mathematics, a science treating of and founded upon the very qualities of forms, magnitudes, and quantities, that are still so absurdly ascribed to the only sense which nature had denied him!

If the eye is the sole source of our ideas of distance, how does it happen that the born-blind daily display such accuracy of perception of distance and relative position in the rapid and almost unerring certainty and safety with which they thread their way through the crowded streets of a city, and judge of the distances of horses, carriages, and other moving obstacles, which they so cautiously avoid? If the eye is the true origin of such ideas, how does it happen, that, after the loss of both eyes and of the functions which they perform, a man still retains accurate notions not only of form, or of distance, but even of colour, which at first sight seems the most dependent of all upon the integrity of the eye? If Milton had been indebted to the eye alone for his sensibility to the harmony of colours, he would have lost that taste when the organ that gave it was destroyed; but, instead of this, the pathetic and affecting address to Light, which he wrote when perfectly blind, shows demonstrably that his sense of colour was as vivid as ever. Nay, even persons born blind, and whose eyes never surveyed the varied hues of nature, have never"theless a distinct conception in their own minds of the quality of colour, which of itself is enough to prove that the eye is only a medium or inlet to, and not the origin of, that sensation. And on this view we can easily explain the ability of the blind to understand and to teach the laws of optics, and even astronomy; and also the exquisitely appropriate descriptions of the beauties of colours by such persons as Dr Blacklock, whose eyes were never open to the light of day." In his poetical productions," says an ingenious writer, "Dr Blacklock alludes to the various beauties of the visible "world, and to the charms and delicacies of colour, with all the 168 propriety, and with all the rapture and enthusiasm, that ever "fired the breast of a poet who had the fullest enjoyment of his "eyesight. Nor was this done mechanically or merely by role; "for having himself put it as a question, How shall we ac

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"count for the same energy, the same transport of description ""exhibited by those on whose minds visible objects were either never impressed, or have been entirely obliterated?' he as68 8 sures us that, however unaccountable this fact may appear, <<< it is no less certain than extraordinary."

Here then we have the most unequivocal testimony in proof of ideas of form, size, distance, and colour, being independent of the mere organ of vision, and of the latter being no more than a medium of communication between external objects and the internal faculties of form, colour, &c., which take cognizance of these qualities. Many philosophers, indeed, and Mr Alison among others, explain such cases as Dr Blacklock's, by referring his delight in colours, not to any distinct original conception of their beauty in his own mind, but to the influence of association in connecting other feelings with the words which he made use of to denote colours; but although it is extremely difficult for us to conceive the nature of the ideas of colour entertained by such persons, yet there is abundant evidence, not only from their own declarations, but from their manifestations, that they do actually possess them.

The simplest way of determining the functions of the eye is one within reach of everybody. It is merely to shut the eye, and then attend to the ideas and functions which still remain ;-vision is at an end; therefore we may say vision is the function of the eye; but the form of our friend's face, the colour of his eyes and hair, the size of his head and nose, and the distance at which he lives, are just as accurately present to the mind as ever they were, and consequently these cannot constitute any part of the function of the eye, which is completely inactive.

Daily and most familiar experience is in strict accordance with this. Every one knows that it is clear and distinct vision, and not a talent for colouring, form, or perspective, that is the invariable accompaniment of a favourably-constituted eye; and that many a weak-sighted man is remarkable for an

Edinburgh Encyclopædia. vol. III., p. 601.

exquisite taste for painting, (colour), drawing, (form), or perspective; while, on the contrary, many examples of the lowest endowment of all of these talents are to be met with in union with the most powerful vision. Mr James Mylne is one instance out of many, known to all our readers, of quick sight co-existing with inability to distinguish some of the most palpable shades of colour with each other. Mr Ferguson, mentioned in Mr Combe's System, is an instance of deficiency in the perception of perspective,-a deficiency which, by the way, seems to be more characteristic of the Chinese than of almost any other people, their mere eyesight being as good as our own. And, in fact, this non-relation of force of vision to taste for colouring, &c., is so well known, that a man would be laughed at by the very children on the street, were he seriously to affirm that looking through a telescope would give such persons as Mr Mylne a high relish for the beauty of colour; which, however, it would do were mere force of vision all that was wanted to improve his talents.

The phenomena of disease support our views equally as those of health. When the eye alone is disorganized, as when the cornea or the crystalline lens has lost its transparency to such a degree as to impede, but without altogether interrupting the passage of the rays of light to the optic nerve, the consequence is not any derangement in our ideas of form or of distance, but simply the imperfect formation of an image on the retina, and the perception by the mind of this imperfection as an existing quality of the image. And in another affection, ophthalmia, which is confined to the eye alone, there is increased sensibility to light; but, unless the disease extends to the brain, and delirium supervene, there is no disturbance of our ideas of colour, of form, or of distance; whereas, in diseases of the brain, the eye remaining sound, false perceptions of distances, features, and relative position, are constantly occurring and leading to fearful mistakes, as is daily seen in mania and in fever. In surgical cases of pressure of the brain from fracture of the skull, the eye all