A more difficult problem remains to be considered, and one which has long exercised the ingenuity of the meta physical physiologists, the cause of single vision with two eyes. When we look at an object we direct both the eyes to it, and of course have an image formed on both the re tina., yet the mind receives only one perception. The question is, whether this depends upon any thing in the organization of the eye, or whether it is merely the result of experience and association ? An opinion has indeed been maintained, and not without a degree of plausibility, that although both the eyes are turned to an object, we in fact only employ one of them at once, the attention being directed to each of them alternately. But against this opinion there is an experiment of Jurin's, who found, that when the eyes are both directed to the same object, it is seen with considerably more vividness than when viewed with only one eye.
Porterfield proceeds upon his general principle, that every object is necessarily seen in the place where it ac tually exists, and as both the eyes see the object in its Veal place, only one object can be perceived. Reid lays it down as a principle of vision, that where the objects fall on what he terms corresponding points of the retina, the eyes can perceive only one object, while Smith, on the contrary, argues that the eyes each of them receive a distinct impression, capable of exciting a distinct idea, but that by habit, and by comparing visible with tangible sensation, we correct the errors of the sight, and finally learn to associate the double impression with a single object.
The opinion of Porterfield we regard as clearly dis proved by the frequent mistakes into which we fall with respect to the position of objects, thus showing that there is not that necessary connexion between their real and their apparent situation which the hypothesis would re quire. Against Reid's doctrine of corresponding points, some direct experiments have been brought forward by Dr. Wells, which seem to prove its fallacy ; and the same ingenious physiologist likewise objects to it, that it is contrary to the analogy of the anatomical structure of the body, that these corresponding points should be both on the right or both on the left side of the retina, because the parts which might be supposed to correspond would be either both without or both within the centres of the eye; and there is an experiment, originally performed by Du Tours, which has been conceived to be decisive against Reid's doctrine. If we look through a tube, at the other end of which are placed two glasses lying over each other, one blue and the other yellow, we perceive a green colour.
Now it is argued, that if we apply a tube to each eye, and at the end of one have a blue, and at the end of the other a yellow glass, we ought, as in the other case, to perceive a green colour, as the images are here conceived to fall upon corresponding points of the retina, where the sen sations will unite as if they had both fallen upon one eye. But we do not find this union to take place ; we, in fact, see one colour at once, first the blue, and then the yellow, or vice versa ; or they sometimes seem to lie one over the other, but they never amalgamate so as to produce the idea of green.
The objections to Smith's doctrine are perhaps still more decisive. There is said to be no instance on record where a person ever had the power of single vision with two eyes, when the eyes were not similarly directed to the object, so that the images might fall upon points similarly situated with respect to the centres of the retina. It is farther stated, that, in Cheselden's case, the patient did not see objects double when ne first received his sight ; and we generally observe that infants and blind persons move the eyes together, as if from some sympathy or natural connexion between them. The effects of intoxi cation have been referred to as favouring Smith's opinion, for here the vision frequently becomes double, depending, as it has been supposed, upon the temporary loss of the power of association. • But the double vision of intoxica tion has been accounted for in a different way, not upon the disturbance of the usual train of associations, but upon the eyes not moving parallel to each other, so that the images fall upon points of the retina which do not corre spond. The same effect is always produced when the eyes are affected by accident or disease, so that they do not move together ; whereas, in insanity, where all the ordinary trains of our ideas are destroyed, provided the eyes be not especially affected, we do not find that double vision exists.
Upon the whole, therefore, we must conclude that the present state of our knowledge does not enable us really to come to any conclusion on this point ; there are some circumstances which lead to the opinion, that the eyes do not actually convey two perceptions at the same time, but that they are always in a state of rapid alternation, an opi nion to which Haller inclines, yet this hypothesis is not without its difficulties; but there are greater objections against the hypothesis of corresponding points as advanced by Reid, and perhaps still greater against the doctrine of Smith.