82 
Dr. Young’s Lecture 
the eye of the libellula ; but there is no difficulty in supposing 
that the means of producing the change of the refractive powers 
of the eye, may be, in different classes of animals, as diver- 
sified as their habits, and the general conformation of their 
organs. 
I beg leave to correct here an observation in my former paper, 
relative to the faint lateral radiations, which I supposed to pro- 
ceed from the margin of the iris. * I find, on further exami- 
nation, that they are occasioned by reflections from the eye- 
lashes. 
XII. I shall now finally recapitulate the principal objects and 
results of the investigation which I have taken the liberty of 
detailing so fully to the Royal Society. First, the determination 
of the refractive power of a variable medium, and its application 
to the constitution of the crystalline lens. Secondly, the con- 
struction of an instrument for ascertaining, upon inspection, the 
exact focal distance of every eye, and the remedy for its imper- 
fections. Thirdly, to show the accurate adjustment of every 
part of the eye, for seeing with distinctness the greatest pos- 
sible extent of objects at the same instant. Fourthly, to mea- 
sure the collective dispersion of coloured rays in the eye. Fifthly, 
by immergirtg the eye in water, to demonstrate that its accom- 
modation does not depend on any change in the curvature of 
the cornea. Sixthly, by confining the eye at the extremities 
of its axis, to prove that no material alteration of its length can 
take place. Seventhly, to examine what inference can be drawn 
from the experiments hitherto made on persons deprived of the 
lens ; to pursue the inquiry, on the principles suggested by Dr. 
Porterfield ; and to confirm his opinion of the utter inabi- 
* Phil. Trans, for 1793. p. 178. 
