770 
but little attended to, and yet it is sufficiently 
evident. When the cornea has been cut, 
operators, disappointed in not finding the 
cataract protruded, keep the eye staring in 
the light, and press the ball of the eye; but 
while the eye is thus exposed to the excite- 
ment of the light, the pupil is contracted, 
and the lens propelled by the action of the 
muscles ; ait still more, by the pressure 
made on the eye-ball, is in danger of burst- 
ing through and tearing the iris. The best 
operators have been in the custom of shut- 
ting the eye-lids the instant the incision was 
made in the cornea; by this means, the eye 
is for a time supported in some degree du- 
ring the violent spasm of the recti muscles, 
and the iris being allowed to dilate, the lens 
is protruded into the anterior chamber of the 
aqueous humour through the pupils, and is 
ready to slip from under the cut cornea, 
when the eye-lids are again opened. By this 
means, if the incision of the cornea is of the 
proper extent, the lens is not extracted, but 
is protruded, by the action of the muscles 
of the eye.” 
In describing the effects of light on 
the retina, we rather expected to have 
found some notice of Buffon’s experi- 
ments on ocular spectra, and ef Dr. 
R. Darwin’s close imitation of them. 
They certainly deserved a place, on ac- 
count of the important physiological ob- 
servations deducible from them. 
On the membrana pupillaris the au- 
thor, after describing its structure, gives 
the following original explanation of its 
use. , 
*« To explain the effect of this membrane, 
then, we have only to consider that it is of 
the nature of the iris to contract its circular 
fibres during the operation of light, so as to 
close or nearly close the pupil ; that, on the 
other hand, the pupil is completely dilated 
through the operation of the radiated fibres 
of the iris in darkness. To the question, 
then, why it is not dilated during the foetal 
state ?_ the answer, I think, is decidedly this: | 
The iris is not loose in the foetal state, it is 
connected and stretched to the middle degree 
of contraction and dilatation by the mem- 
brana pupillaris. Were the iris ina full state 
of @pntfaction, during the life of the fetus, 
it could not receive its full nourishment, pro- 
er degree of extension, and due powers ; 
fot being preserved stationary and extended, 
the disposition to contraction, which it must 
have when the retina is without excitement, 
4s counteracted, until it is about to receive, 
by the birth of the child, that degree of ex- 
citement which is to keep up the prepon- 
derance towards the contracted state of the 
pupil.” 
The much agitated controversy con- 
cerning the method by which the eye 
MEDICINE, SURGERY, ANATOMY, xc. 
adapts itself to different focal distances, 
the imagined encrease of convexity in 
the cornea, the supposed muscularity of 
the crystalline, and the other questions 
relating to this subject, are collected 
with industry, and detailed with fairness. 
lt is a little surprising, however, that 
the author, in giving an abstract of Dr. 
Young’s most ingenious paper (the 
Bakerian Lecture, in the Transactions 
for 1801) should have overlooked the 
most important experiment of all (we 
mean that of enclosing the eye in water 
behind a convex lens, and ascertaining 
the same change of focal distance) and 
one on which the inventor himself justly 
lays the greatest stress. 
Mr. Bell appears equally deficient in 
the conclusions which he draws from all 
the experiments on this subject. After 
acknowledging himself entirely undeter- 
mined which hypothesis to adopt, he 
thinks proper to reject all, and from the 
following reasoning. 
«« T have often doubted, whether these ex- 
perimenters were not in search of the explae 
nation of an eflect which has no existence. 
I have never been able to determine, why a 
very slight degree of convexity in the cornea 
of a short-sighted eye should be so perma- 
nent during a whole life-time, notwithstand- 
ing-the perfect elasticity of the cornea, and 
its being so adapted as to alter its convexity 
by theaction of the muscles. Again, a near- 
sighted person, with the assistance of a con- 
cave glass, can command the objects to the 
distance of some miles, and with the glass 
still held to the eye, can see minute objects 
within three inches of theeye. Now, I can- 
not conceive how the concave glass should 
give so greata range to the sight; as there 
can be no change in the glass, it must be the 
eye which adapts itself to the variety of dis- 
tances ; yet, without the glass it cannot com- 
mand the perfect vision of objects for a few 
feet. Again, a short-sighted person sees an 
object distinctly at three inches from his eye; 
at twelve feet, less distinctly ; and when he 
looks upon the object at twelve feet, the ob- 
jects beyond it are confused, just as in other 
men’s eyes; but when he directs his atten- 
tion to the more remote objects, those nearer 
become indistinct. Now this indistinetness 
of the object, seen when he examines nar- 
rowly the object beyond them, would argue 
(did we admit this muscular power in the eye 
of adapting itself to objects), that the cornea 
or the lens has become less convex, were we 
not previously convinced that the utmost 
powers of the eye could not bring the object 
at the distance of twelve feet, or any other 
intermediate distance, to be more distinctly 
seen than the fixed and permanent constitu- 
tion of the eye admits.” 
