NUNNELEY, ON THE CRYSTALLINE LENS. 137 
human ingenuity to imitate. Hence, while its importance in 
health, the great changes which it undergoes in disease, and 
the skill and knowledge requisite for its treatment and 
removal, have always rendered it an object of great interest 
to the anatomist and surgeon—its perfect action in con- 
verging the rays of light to a focus upon the retina, so as 
practically, if not absolutely, to overcome the aberration of 
sphericity, has caused its form, structure, and density to be 
matters of the closest investigation and calculation by op- 
ticians and mathematicians. The difficulties of the inquiry 
have been at least equal to their importance, so that as much, 
if not more, controversy and difference of opinion have 
existed as to the structure of the crystalline lens, as of any 
part of the eye. 
As is well known, the lens is partially imbedded in the 
anterior surface of the vitreous humour, where it is held in 
situ by the elastic suspensory ligament. It lies immediately 
behind the iris at the juncture of the anterior with the middle 
third of the globe. In man, mammalia, birds, and reptiles it 
is a double convex lens, of which the posterior surface is con- 
siderably the more convex, particularly i the two former 
classes. To the exact proportions which the curves of the two 
surfaces bear to each other, opticians have necessarily at- 
tached great importance. Many experiments have been made, 
and far more numerous elaborate calculations entered upon 
for determing it. Such inquiries are perhaps of more 
interest and importance to the optician than to the anatomist 
and surgeon; and from the nature and structure of the lens, 
when made experimentally, are not susceptible of absolute 
accuracy ; for it is almost impossible to measure, with mathe- 
matical precision, the curves of a small, delicate, yielding 
substance, like the lens. Moreover, it is beyond doubt, that 
not only do the two surfaces differ somewhat in different im- 
dividuals, but they vary very much in the same person at 
different periods of life. There is a gradual flattenmg of the 
surfaces, with an increasing density of the substance of the 
lens, as age advances. In the new-born infant the lens is as 
soft as rather thin jelly ; in old age as firm as suet. In infancy 
the lens is comparatively convex to its form in the aged. 
Fig. 1 (plate) shows their varymg forms—a in infancy, 6 in 
the adult, ¢c inoldage. However, notwithstanding these diffi- 
culties and changes, much interest and importance is attached 
to the determining, with as much precision as the nature of the 
case admits of, the form of the curves and the density of the 
material forming the lens. So far as I am aware, Petit, in the 
earlier half of the last century, is the only person who has care- 
VOL. VI. M 
