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20 be in strict life and action, whilst the mind itself should be in vigour and ability, so as to realize a simultaneous action or recognition of the outward object, and commix it with all the store of previous possessions of knowledge.

Occasionally it occurs that the object arrives at the retina without rendering any knowledge or report to the brain, owing either to some defection in the action of the optic nerve, or unsoundness of cerebral power, and then the sensation or feeling of sight (if we may be allowed the expression), is wholly dull and immobile, and, of course, no perception is realized.

Sometimes persons, as we know, are in abstract and deep thought, when the object has arrived on the retina, and they appear not to have seen anything; but, after a time, they will tell you they felt the arrival of the object or image on the retina and perceived its nature, which accorded or varied with their previous knowledge or experience. It is not needful for us to quote instances when the optic nerve is sensitive, and even the brain acknowledges the presence of light, and yet the objects find no place in the mind.

It is sometimes a curious inquiry, how it is that having two eyes, each perfectly adapted for vision, we yet see but one object, or rather have but one picture on the retina. The most simple answer seems, that if both eyes are directed to one object, the two optic lines will concur at the same point of the object, and the two images (one to each eye) will be simultaneously referred by the retina to the single point of concurrence of both optic lines. Although each eye may be said to have an image, yet the sensation of the two retinæ are of a correspondent nature, and being made in the same manner and time, they must harmonize with each other in respect to this triple unity of nature, manner, and time. After we leave the mechanical powers and