ascertained that we derive nothing from the eye whatever but sensations of colour—that the idea of extension [he means in its three dimensions] is derived from sensations not in the eye, but in the muscular part of our frame."[1] Thus, contrary to what Mr Bailey affirms, these two philosophers limit the office of vision to the perception of mere colour or difference of colour, denying to the eye the original perception of extension in any dimension whatever. In their estimation, the intuition of space is no more involved in our perception of different colours than it is involved in our perception of different smells or different sounds. Dr Brown's doctrine, in which Mr Mill seems to concur, is, that the perception of superficial extension no more results from a certain expanse of the optic nerve being affected by a variety of colours than it results from a certain expanse of the olfactory nerve being affected by a variety of odours.[2] So much for Mr Bailey's assertion, that all philosophers admit the perception of extension in two dimensions.
But, of course, our main business is with the expression of his own opinion. In rebutting our charge, he maintains that "the visibility of angular distance (that is of extension laterally) is assumed, by implication, as part of Berkeley's doctrine, in almost every chapter of my book."—( Letter, p. 13.) That word almost is a provident saving clause; for we undertake to show that not only is the very reverse assumed, by implication, as part of Berkeley's doctrine, in the single chapter to which we confined our remarks, but that, in another part of his work, it is expressly avowed as the only alternative by which, in the author's opinion, Berkeley's consistency can be preserved.
At the outset of his enquiry, Mr Bailey divides his discussion into two branches: first, Whether objects are originally seen to be external, or at any distance at all from the sight; and, secondly, Supposing it admitted that they are seen to be external, or at some distance from the sight, whether they are all seen in the same plane, or equally near. It was to the former of these questions that we exclusively confined our remarks;[3] and it was in reference to it, and to an important argument evolved by Mr Bailey in the course of its discussion, that we charged him with fathering on Berkeley the doctrine which he now disavows as his interpretation of the bishop's opinion. He further disputes the relevancy of the question about our perception of lateral extension, and maintains that distance in a direction from the percipient, or what we should call protensive distance, is the only matter in dispute; and that it is a misconception of the scope of Berkeley's essay to imagine otherwise. The relevancy of the question shall be disposed of afterwards. In the mean time, the question at issue is, Can the allegation which we have laid to Mr
- ↑ Mill's Analysis, vol. i. p. 73.
- ↑ This reasoning of Dr Brown's is founded upon an assumed analogy between the structure of the optic nerve, and the structure of the olfactory nerves and other sensitive nerves, and is completely disproved by the physiological observations of Treviranus, who has shown that no such analogy exists: that the ends of the nervous fibres in the retina being elevated into distinct separate papillæ, enable us to perceive the extension, and discriminate the position of visible bodies,—while the nerves of the other senses being less delicately defined, are not fitted to furnish us with any such perception, or to aid us in making any such discrimination. See Müller's Physiology, translated by W. Baly, M.D. , vol. ii. pp. 1073, 1074. Although the application of Treviranus's discovery to the refutation of Dr Brown's reasoning is our own, we may remark, in justice to an eminent philosopher, that it was Sir William Hamilton who first directed our attention to the fact as established by that great physiologist.
- ↑ Mr Bailey seems disposed to carp at us for having confined our remarks to this first question, and for not having given a more complete review of his book. But the reason why we cut short our critique is obvious; for if it be proved, as we believe it can, that objects are originally seen at no distance whatever from the sight, it becomes quite superfluous to enquire what appearance they would present if originally seen at some distance from the sight. The way in which we disposed of the first question, however imperfect our treatment of it may have been, necessarily prevented us from entering upon the second; and our review, with all its deficiencies, was thus a complete review of his book, though not a review of his complete book.
