Since the motions of the eyes are in every instance congruous, from the instant of birth, an associated tendency thereto is generated sooner, and more firmly established, than perhaps in any other case. As therefore the external influence of light, by growing languid, comes to have less and less effect perpetually for this purpose, so the associated tendency grows stronger and stronger; and the sum total of both may perhaps be always about equal.
However, this congruity does not seem to be so great in children as in adults, who can direct their optic axes by a perfectly voluntary power to the object which they intend to view with attention. We must examine therefore, how the almost perfect congruity becomes an entirely perfect one.
Now, here we are to observe, that the almost perfect congruity begets an almost perfect correspondency in the points of the retina; and that hence it will follow by degrees, that the least deviation from perfect congruity will occasion double vision, and confusion. But these are unpleasant and inconvenient, whereas single and distinct vision is pleasant and convenient. Whence every recurrency of the last will tend to confirm it, of the first to exclude it, from principles already laid down, Prop. XXII. Cor. I. The child will therefore come to a perfect congruity at last, i.e. to direct his optic axes precisely to the point which he intends to view. The voluntary power of suiting the internal motions to the distance must be supposed to grow perfect about the same time.
It agrees with this method of reasoning, that persons who lose the sight of one eye in their childhood or youth, though long after the external action of light has lost its efficacy for making the motions congruous, generally squint a little with that eye.
The persons who squint, preserving the sight of the squinting eye, are obliged to move their eyes in a congruous manner, for the same reason as others, viz. to avoid double vision, though the position be incongruous.
The constant action of the elevator palpebræ superioris in adults seems to be entirely from custom, i.e. association, being kept up in a more particular manner by the variety of visible objects, which engage our attention during the course of the day.
It is generally some time before children get the voluntary power of shutting the eyes gently, or of shutting one and not the other. They can shut them with force soon, this action recurring often from motes in the eyes, pain of any kind, &c. The procedure here is of the same kind as in other voluntary actions.
The internal motions depend originally upon the greater or less quantity of light which falls upon the cornea and iris, as has
