Mr James Mill, an author whom Mr Bailey frequently quotes with approbation, and in confirmation of his own views, is equally explicit. He maintains, in the plainest terms, that the eye has no intuition of space, or of the reciprocal outness of visible objects. "Philosophy," says he, "has 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."[29] 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.[30] So much for Mr Bailey's assertion, that _all_ philosophers admit the perception of extension in two dimensions. [29] Mill's Analysis, vol. i. p. 73. [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.