right-hand candle on the right retina, and that of the left-hand candle on the left retina, will each produce its proper image. See Smith’s Optics, Rem. 526.
But here two questions may be asked; First, Why single objects appear the same to one as to both eyes, allowing for the diminution of brightness, since, in the first case, there is one picture only, in the last two. Ought not every single object to appear single to one eye, and double to both?
Secondly, How can one object appear like two to both eyes, since, however the eyes be directed or distorted, it can make but two pictures, whereas two objects make four, viz. two in each eye?
It is evident, that the difficulty is the same in both these questions. And it seems to be a sufficient answer to allege, that impressions so much alike, and which are so constantly made together, as those upon the corresponding portions of the two retinas, must unite into one entirely in the brain, and produce the same effect in kind, though somewhat different in degree, as one alone. And thus whether we see with one eye or both, hear with one ear or both, the impression, on the common sensory in the brain is the same in kind; and therefore, if the first be called single, the other must also.
But it deserves particular attention here, that the optic nerves of men, and such other animals as look the same way with both eyes, unite in the sella turcica in a ganglion, or little brain, as one may call it, peculiar to themselves; and that the associations between synchronous impressions on the two retinas must be made sooner, and cemented stronger, on this account; also that they ought to have a much greater power over one another’s images, than in any other part of the body. And thus an impression made on the right eye alone by a single object may propagate itself into the left, and there raise up an image almost equal in vividness to itself: and consequently, when we see with one eye only, we may, however, have pictures in both eyes; and when we see a single object, with our eyes directed to one at a different distance, we may have four pictures, viz. two from direct impression in parts that do not correspond, and two others from association in parts that do. And thus both the foregoing questions may be answered, in a manner that leaves no doubt or hesitation.
It is evident, that gay colours, of all kinds, are a principal source of pleasure to young children; and they seem to strike them more particularly, when mixed together in various ways.
