The optic nerves possess the remarkable power of conveying to the individual the sen sation of a single impression only, while a separate impression affects each nerve simul taneously, so that although a perfect picture of the object he depicted on each retina severally, nevertheless to the. spectator it appears to be single (as is really the case); or to speak still more intelligibly, a spectator sees an object single, although he looks at it with two eyes.
This property would seem to ,belong in an especial manner to the optic nerves, and. inas rnuch as the second pair differ from all the others in possessing a chiasma, there is so far a presumption, that the unity of sensation mani fested by the optic nerves depends on the chiasma.
The idea that single vision may be explained by a partial decussati on in the chiasma originated (upon theoretical grounds) with Newton ; it has since been adopted by Wollaston, Solly, and others, and many facts have been from time to time brought forward in its favour. The hypothesis may be thus enunciated. " Each tractus opticus sends some filaments across the chiasma to form the inner part of the opposite optic nerve, while its outer filaments continue on to form the outer part of the optic nerve of its own side; the same armngement of the filaments prevails to the retina, so that the right side of each retina comes from the right tractus opticus, and the left side of each retina from the left tractus opticus ; if then in vision the pictures of an object be depicted simultaneously either on the right sides of the two retinw, or on the left sides of the two retinze, the impressions in either case will be communicated to one and the same tractus opti cus; such impressions will of course be referred to one and the same side of the brain, and they will therefore produce the sensation of a single impression only, although in reality two several impressions affect the retinm; the unity of sensation depending on the fact that the two impressions are referred to the same silk of the brain, and not to opposite sides of the brain as occurs in other cases." Wollaston adopted this hypothesis in conse quence of the ready explanation it affords of certain cases of visus dimidiaius: he on several occasions in his own person experienced a tem porary defect of vision, in consequence of which one and the same lateral half of every object became invisible, whilst he still continued to see the other half distinctly ; and the same amount of blindness sUbsisted whether he em ployed one or both eyes in looking at the object.
Examples of the same form of amaurosis are of no very unfrequent occurrence, and according to the theoretical notions just propounded this partial loss of vision originates in some func tional affection of one tractus opticus whilst the other remains healthy ; those parts of the two retinw which derive their origin from thejaulty tractus being supposed to labour under a tem porary amaurosis, while all other parts retain their ordinary sensibility.
In further support of this hypothesis, Mul ler's researches have convinced him that in man single vision by the two eyes occurs only when certain parts of the two retinw are affected simultaneously, and that under other circum stances double vision ensues. This conclusion has been arrived at, chiefly from the results of experiments upon the eye-ball. Thus, when the eye-lids are closed in a dark room, if pressure be applied deeply to the eye so as to affect the retina, luminous spectra are produced. When certain parts of the two eye-balls are subjected to pressure at the same time, a single spectrum occurs, and when other parts of the two eye-balls are pressed upon simultaneously two spectra appear. Those parts of the two retinae which in the above experiment furnish a single spectrum are styled " identical," in con sequence of their identity of sensation ; and those parts which produce two spectra are de nominated " non-identical,"forobvious reasons. Some of the conditions under which single and double vision respectively take place, seem to be confirmatory of this doctrine of " identical " and " non-identi`cal " parts in the two retinae; thus double vision is a common consequence of any cause having a tendency to disturb the relative directions of the optic axes : for exam ple, diplopia frequently occurs in cases of strabisinus, and double vision may (according to Afuller) be produced in perfectly healthy eyes by a simple experiment : if a spectator fix his eyes upon an object and then press on one of them in such manner as to alter the direction of its a.xis, the object which at first seemed single will assume a double appearance. These phenomena admit of explanation on the suppo sition that in consequence of the distortion of the axes of the eyes, the visual impressions take effect on " non-identical " parts of the two retinx, which therefore propagate two impres sions instead of one to the sensorium.