138 NUNNELLY, ON THE CRYSTALLINE LENS. 
fully and extensively experimentally investigated the first of 
these problems ; for though many persons have since then, and 
particularly Dr. Porterfield and Dr. Thomas Young, written 
elaborately on the subject, it would appear their investiga- 
tions have rather been theoretical and calculative, based upon 
Petit’s measurements, than upon original experiment, except 
perhaps Dr. Young; and from his statement I am unable to 
determine whether he exclusively depended upon Petit’s 
figures or not.* According to Petit the diameter of the lens 
is about four or four and a half lines, and its axis two lines. 
“The diameter of the sphere of which the anterior segment 
is a part being from six to twelve lines, but most commonly 
about seven and a quarter or eight lines; whereas the dia- 
meter of the sphere of which its posterior segment is a part 
is commonly only about five or six lines.’”+ According to 
the calculations of Dr. Young the radius of the anterior 
surface of the lens is ‘30, of the posterior surface *22 of an inch. 
The following table will show the result of my measure- 
ments of the lenses of different creatures. I give them only 
as approximations to accuracy; for though I have taken every 
care that I could exercise to ensure as much correctness as 
possible, as I before said, I believe it to be impossible to 
measure any single lens with absolute precision ; the result of 
the whole must be looked at, and this, I think, will not be 
far from the truth. One proof I think of the general cor- 
rectness of the measurements, is the curious fact shown by 
them, that the proportion of the curve of the posterior 
surface to the diameter of the lens, is far more uniform than 
that of the anterior surface. The radius of the posterior 
curve differs very little in any of the creatures from the half 
of the diameter of the lens, so that in fact the posterior 
surface of the lens is nearly, but not absolutely, a hemisphere, 
while the anterior is a segment of a much larger sphere, it 
being on this surface that the variations at different ages in 
the individual, and different creatures, take place much more 
than in the posterior surface of the lens—a point of no little 
importance to the ophthalmic surgeon. The table also shows, 
that as we descend from man to fish, there is a gradual in- 
crease in the convexity of the lens until in them it is a true 
sphere, and that this increase in the sphericity is not neces- 
sarily accompanied by an increase in the density of the lens, 
as a reference to the table of the specifie gravities of the lens 
will show. The measurements are given in parts of an English 
inch. 
* «Miscellaneous Works of Dr. Thomas Young.’ By Dr. Peacock. 
Vol. i, Nos. 1, 2. 
+ Porterfield’s ‘Treatise on the Hye,’ vol. i, p. 231. 
