Page:Proceedings of the Royal Society of London Vol 1.djvu/57

Rh in some eyes, and certain instances of oblique vision may be duly ac- counted for; also the different refractive powers of the crystalline lens at the centre and near its surface, the former after death being to that of water in the proportion of 21 to 20, and gradually decreasing till at the surface it becomes equal to that of the surrounding medium, thus producing a mean refraction for the whole lens, considered as a body of equal density, in the proportion of 14 to 13 when compared with that of water. We also find here that the whole extent of perfect vision is little more than 10°, or more strictly speaking, that the imperfection begins within a degree or two of the visual axis, and that at the distance of 10° or 15° it becomes nearly stationary, until at a still greater distance vision is wholly extinguished; but that the motion of the eye, at the same time, has a range of about 55° in every direction, so that the field of perfect vision, in succession, is by this motion extended to a circle of 110° diameter. The advantage also of the spherical form of the eye, not only for motion but also for vision, is illustrated by diagrams. These few observations are here inserted not as a just delineation of this important part of the lecture, which cannot be condensed within our limits, but as a few examples of the sort of information tbe reader may expect to derive from it.

In a following section the author proceeds to inquire how great are the changes which the eye admits, and what degree of alteration in its proportions will be necessary for these changes, on various suppositions;-1. A change in the radius of the cornea. 2. A change in the distance of the crystalline lens from the retina. 3. These two causes acting conjointly; and 4. Some alteration in the figure of the lens itself. A minute inquiry follows next, which of these changes actually takes place in nature: and here a variety of experiments are mentioned, contrived for the purpose of deciding on the truth of each of these suppositions. The object of the first series of these experiments, the results of which were directly inferred from the effects of immerging the eye in water, is to ascertain the curvature of the cornea in all circumstances; and from these results it appears that the cornea is not concerned in the accommodation of the eye. A similar investigation is instituted to inquire whether any alteration in the length of the axis of the eye, which would affect the distance of the lens from the retina, actually takes place in nature. And here, too, the results are, that it is highly improbable that any material change in the length of this axis is ever produced, and that it is almost impossible to conceive by what power such a change could be effected. The opinion of the joint operation of these two causes, which had derived great respectability from the ingenious and elegant manner in which it had been treated by Dr. Olbers of Bremen, and from being the result of the investigation of Mr. Home and the late Mr. Ramsden, is, lastly, shown to be inconsistent with the experiments related in this рaper.

We now come to the important section, in which the author inquires into the pretensions of the crystalline lens to the power of altering the focal length of the eye. The grand objection to the