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Thus the admirable phenomenon of vision is effected inman, and in the greater number of animals, by means ofconical surfaces traced in space, and in our eye ; theagent being rays of light scattered in every direction, bybodies luminous, either m their own nature, or from re-flecting the light of luminous bodies.
or picture painted, on the retina, as is described in the text. Theyhave also ascertained that the optic nerve, in a healthy state, andthe brain, are both necessary to correct vision. They have provedtoo, that when the optic nerve and the brain are in a sound state,when individuals are awake, and when the above described im-pression is made on the retina, vision, or a sensation of sight, isproduced; or we see the object, the image of which is impressed onthe retina. But there is nothing in the connection between thepicture on the retina, and our consciousness of perceiving the exter-nal object, to warrant the assertion, that this impression or thispicture, is transmitted to the optic nerve, and subsequently transmit-ted to the brain. It is quite plain that the image is not transmitted,for that is altogether an optical phenomenon, and neither the nervenor the brain has, as far as we know, any facility for either receiv-ing or transmitting optical impressions. The picture on the re-tina is the ultimate physical circumstances which physiologists haveyet traced when vision occurs ; and between that and actual sensa-tion and perception,—or the mental act of seeing, all is darkness.We know that there are impressions on the retina ; we know that inordinary cases when they are received, vision follows; but we knownot how the bodily impression is connected with the mental percep-tion, and to account for it, by further impressions on the optic nerve,by impressions on the brain, and by this being the seat of intelli-gence, only serves to keep our ignorance out of view, and attemptsto explain the incomprehensible connection between the impressionon the retina, and the sensation of sight, by another equally in-comprehensible connection, viz. an impression on the brain with thissensation, of which connection, however, we have no knowledge what-ever. There are in every direction some bounds to our knowledge,which, although they continually recede as investigation proceeds,can never be passed;—which equally exist, though their extent isdifferent, for the most ignorant savage and the most enlightenedphilosopher ; and at which every man feeling himself suddenly ar-rested, and being unable to explain the connection between certainphenomena, feels the sentiment of wonder, and is compelled to re-verence a Power, the ways of which, he is thus made sensible, areinscrutable. Tr.