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effects which have been wrought in themselves, and that unaffected they can be the medium of no knowledge. Effects not only demand causes, but causes efficiently present in them, interpenetrating them. The last, the immediate cause is inseparable from the effect. Now the vibrations of light and sound are the agents and the only agents that reach these organs, and it is a matter of experience that perception is immediately dependent on these agents as they penetrate into and work their changes on the organs of sense. Each organ is obviously fitted for the action of its own agent, and every interference with these internal adjustments destroys perception wholly or in part. While, therefore, our necessary beliefs demand an immediate effect on the organ of perception, experience clearly points out the agents of this effect, and the contrivance by which it is wrought.

The purely intellectual character of sight, the extent to which the eye is an unconscious, translucent medium of the mind, is shown by the number, delicacy, variety, and furtive character of the judgments inextricably involved in vision. The earlier years of life are evidently busily employed in learning to see, not in the scientific but in the familiar use of the word. These facts harmonize with the further recorded fact, that the eyes of one couched in mature life, seemed to report all objects under the analogy of touch; that is, as directly in contact with the organ of vision.* These spaces, greater and less, which the educated eye now reveals; this opening up and spreading out of the universe before it, this unsearchable depth, this height, this

* The case referred to is that described by Voltaire. The operation was performed by Cheselden on a lad who had been born blind. " It was long before the patient could distinguish objects by size, distance or shape. Several other like cases have been reported." See Diderot, by John Morley, p. 54.