When we have attained a voluntary power over the external motions of our eyes, so as to direct them to objects at pleasure, we always do it in such a manner, as that the same points of objects fall upon correspondent points of the two retinas. And this correspondence between the respective points of the retinas is permanent and invariable. Thus the central points, or those where the optic axes terminate, always correspond; a certain point on the right side of the right retina always corresponds (whatever object we view) to another certain point on the right side of the left retina, equally distant from the centre with it, &c. Hence if the optic axes be directed to the object A, the picture made by it on the right retina corresponds to that made on the left; whereas the impressions made by two similar objects, A and B upon the two retinas, do not correspond. The impressions, therefore, that are made upon portions of the retinas, which do or do not correspond, are the associated criterions of single and double vision. For I here suppose, that the common appearances of a single object, and two similar ones, are respectively called single and double vision.
Let us now inquire into the fallacies which these associated criterions may occasion.
First, then, When a person directs his eyes by a voluntary power to a point nearer or farther off than the object which he views, so as to make the pictures of the object fall upon the points of the two retinas, that do not correspond, this object will appear double. The same thing happens when one eye is distorted by a spasm, when persons lose the voluntary power of directing their optic axes to objects, and in general whenever the pictures, which the object imprints on the two retinas, fall upon points that do not correspond.
It resembles this, and illustrates it, that if we cross the fingers, and roll a pea between two sides, which are not contiguous naturally, it feels like two peas.
Secondly, After a person, whose eye is distorted by a spasm, has seen double for a certain time, this ceases, and he gains the power of seeing single again, provided the distortion remain fixed to a certain degree. For the association between the points of the two retinas, which corresponded formerly, grows weaker by degrees; a new one also between points, that now correspond, takes place, and grows stronger perpetually.
Thirdly, If two lighted candles, of equal height, be viewed at the distance of two or three feet from the eyes, so that the picture of the right-hand candle on the left retina shall correspond to that of the left-hand candle on the right retina, only one image will be produced by these two corresponding pictures. But the two pictures which do not correspond, viz. that of the
