VISION

In Optics, the faculty of seeing. Philosophers have disputed much respecting the means of vision, and its seat in the eye. The Platonists and Stoics held vision to be effected by the emi ion of rays out of the eyes. The Epicureans supposed vision to be performed by the emanation of images or corporeal species from objects to the eye; and the Peripatetics differed in opinion from the Epicureans by supposing the species received by the eye to be incorporeal. Modern philosophers agree in referring the cause of vision to the impre ions of light on the eye; but whether this impre ion is made by material particle? emanating from a self luminous body, and reflected from visible objects, or by the medium of a fluid of great elasticity exciting the eye by the vibrations communicated to it by the luminous body, is a question which is still undetermined. {See Light.) In what manner the eye, or the particular part of it affected by the light, conveys to the brain the impre ions it receives from the luminous rays, is a question which philosophy has not yet solved, and will probably never be able to solve. Seat of Vision. — The retina of the eye, on which an inverted Image of extemel objects is formed, has usually 1305 VITAL VESSELS. been regarded as the seat of vision; but this opinion was long ago brought into doubt by the accidental discovery of Mariotte, that the base of the optic nerve is insensible to the rays of light, and consequently incapable of convej'ing to the brain the impre ion of vision. But the retina, being only the extension or contraction of the optic nerve, Mariotte inferred that if the former were the true seat of vision, that part of it al the entrance oi the nerve, and where it exists in the greatest quantity, and where precisely vision is wanted, ought to be at least as sensible to the rays of light as the other parts. Mariotte, therefore, concluded that the choroid coat which lies immediately below the retina, and lines the whole bottom of the eye excepting the place at which the optic nerve is inserted, is the real seat and cause of vision. {Prieitley on Vision, Light, and Colours, p. 722.; or Hutton's Dictionary.) The opacity of the choroid ' coat, and the transparency of the retina, which render it [ an unfit ground for the reception of images, furnish arguments in support of this view; and an interesting fact was observed by Sir D. Brewster which points to the same conclusion. He observed that in young persons the choroid coat, which is generally supposed to be black, and to grow fainter by age, reflects a brilliant crimson colour, like that of <logs and other animals. Hence, if the retina is affected by rays which pa through it, this crimson light, which must nece arily be transmitted by it, ought to excite the sensation of crimson, which he found not to be the case. (" Optics," Cab. Cvcl. p. 291.) M. Lehot, a French writer, has recently attempted to prove that the seat of vision is in the vitreous humour; and that instead of seeing a flat picture of objects, we actually see an image of three dimensions. To produce this effect, he supposes that the retina sends out a number of small nervous filaments, which extend into the vitreous humour, and convey to the brain the impre ions of all parts of the image. (Id. p. 292. For further information on this subject, see Jwrm's E ay on Distinct and Indistinct Vision, in Smith's Optics; Robins' Tracts, vol.ii.; a paper On Vision by Dr. T. Young, Phil. Trans. 1793; Horn on the Seat of Vision, 8vo. 1813.) See Optics. VISION, [s. 1318]