37 



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 per- 

 fect 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 1 10 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 the 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 experi- 

 ments, the results of which were directly inferred from the effects of 

 immerging the eye in water, is to ascertain the curvature of the cor- 

 nea 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 im- 

 possible 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 

 paper. 



We now come to the important section, in which the author in- 

 quires into the pretensions of the crystalline lens to the power of 

 altering the focal length of the eye. The grand objection to the 



