24 } 
Dr. Young’s Lecture 
Gottingen in 1 jg6, * and also in an Essay presented last year 
to this Society. -f About three months ago, I was induced to 
resume the subject, by perusing Dr. Porterfield’s paper on 
the internal motions of the eye ; ^ and I have very unexpectedly 
made some observations, which I think I may venture to say, 
appear to be finally conclusive in favour of my former opinion, 
as far as that opinion attributed to the lens a power of changing 
its figure. At the same time, I must remark, that every per- 
son who has been engaged in experiments of this nature, will 
be aware of the extreme delicacy and precaution requisite, both 
in conducting them, and in drawing inferences from them ; and 
will also readily allow, that no apology is necessary for the 
fallacies which have misled many others, as Well as myself, in 
the application of those experiments to optical and physiolo- 
gical determinations. 
II. Besides the inquiry respecting the accommodation of the 
eye to different distances, I shall have occasion to notice some 
other particulars relative to its functions ; and I shall begin 
with a general consideration of the sense of vision.: I shall 
then enumerate some dioptrical propositions subservient to my 
purposes, and describe an instrument for readily ascertaining 
the focal distance of the eye. On these foundations, I sfia.ll 
investigate the dimensions and refractive powers of the human 
eye in its quiescent state ; and the form and magnitude of the 
picture which is delineated on the retina. I shall next 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 the various suppositions that are principally 
* De Corporis humani Viribus conservatricibus, p. 68. 
f Phil. Trans, for , p. 146. J Edinb. Med. Essays, Vol> IV, p. J 24 . 
