[30] 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 the great physiologist.
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;[31] 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 Bailey's charge be proved to be the fact, or not?