An American text-book of physiology . it will befound that the retina, if removed and examined under red light, will show theimage of the luminous object impressed ujion it by thebleaching of the visual purple. If the retina be treated with a 4 per cent, solution ofalum, the restoration of the visual purple will be pre-vented, and the so-called ^ optogram will be, as pho-tographers say, fixed. ^Fig. in eye Figure 236 shows the ai)iiearance of a rabbits retina of rabbit (Kiihne). , • , , /> • i i i • i on whicli the optogram oi a wnidow has been nupressed. Although the chemical
An American text-book of physiology . it will befound that the retina, if removed and examined under red light, will show theimage of the luminous object impressed ujion it by thebleaching of the visual purple. If the retina be treated with a 4 per cent, solution ofalum, the restoration of the visual purple will be pre-vented, and the so-called ^ optogram will be, as pho-tographers say, fixed. ^Fig. in eye Figure 236 shows the ai)iiearance of a rabbits retina of rabbit (Kiihne). , • , , /> • i i i • i on whicli the optogram oi a wnidow has been nupressed. Although the chemical changes in the visual purple under the influence of light seem, at first sight, to afford an cxjilanation of the transformation of the vibrations of the luminiferous ether into a stimulation for the optic nerve, yet the fact that vision is most distinct in the fovea centralis of the retina, which, ^ Salzer: Wiener Sitzungsberichle, 1880. Bd. Ixxxi. S. 3. Kiihne: Unlersuchungen a. d. phys. Inst. d. UniversUdt Heidelbeig, i. THE SENSE OF VISION. Ill as it contains lU) rods, is destitute ol visual purple, makes it impossible torejiard this colorinj>; matter as essential to vision. The most probable theoryof its tunotion is perhaps that which connects it with the adaptation of theeye to varying amounts of ligiit, as described on p. 772. In addition to the above-mentioned movements of the ])igment epitheliumcells under the influence of light, certain changes in the retinal cones of frogsand fishes have been observed.^ The change consists in a shortening and thick-ening of the inner portion of the cones when illuminated, but the relation ofthe ])henomenon to vision has not been explained. Like most of the living tissues of the body, the retina is the seat of electri-cal currents. In repose the fibres of the optic nerve are said to be positive inrelation to the layer of rods and cones. When light falls upon the retina thiscurrent is at first increased and then diminished
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