Of the internal motions I will consider the contraction of the greater and lesser rings together, as proceeding from the same causes, and being cotemporaneous; and, for the same reasons, the contraction of the radiated fibres, and ciliar ligament, together also. By the first the eye is fitted for distinct vision at small distances, by the other at great ones. Thus, let us suppose a candle to be brought nearer and nearer to the child’s eye. It is evident, that the quantity of light which falls upon the eye will grow greater and greater. It will therefore agitate all the circular fibres of the iris more powerfully, and particularly the greater and lesser rings; i.e. it will bend the cornea into a greater convexity, bring the origin of the ciliar ligament nearer to its insertion in the capsula of the crystalline, i.e. suffer the capsula to become more convex also, and narrow the pupil, i.e. lessen the radius of dissipation. The image of the candle upon the retina may therefore continue to be distinct, as it approaches, by this mechanical influence of light upon the eye. And, for the same reasons, it may continue distinct, as it recedes. But there are limits on both hands. And thus the conformations of the eye necessary for distinct vision, according to Dr. Jurin’s most accurate account of this matter, are brought about automatically, and suitably to the general theory of these papers.
However, it is also probable, that the light which passes in at the pupil, has great efficacy in contracting both the greater and lesser rings, as may be concluded from the immobility of the pupil in a gutta serena; also because, on this supposition, the light, which passes in at the pupil must, by contracting the lesser ring, become a check and guard against its own too free admission, which is agreeable to the tenor of nature in like instances. The retina extends to the greater ring, and may send some nervous fibres to it, and even to the iris.
One or both of these actions of light seem to increase the secretion and circulation of the aqueous humour in new-born children, so as to fit the eye for vision, which it is not, through the deficiency and muddiness of the aqueous humour, till some time after birth. This again may be considered as a circumstance that favours our present conjectures.
As to the radiated fibres of the iris, and the ciliar ligament, they do not seem so much to be excited to action by any external influence, as to be kept in a state of constant small activity by the vibrations which descend from the brain. When therefore the rings are relaxed, the radiated fibres will open the pupil, and the ciliar ligament draw out the capsula of the crystalline to a flatter shape, and thus suit the eye to obscure and distant objects.
