Mr . Home's Lecture 
4 
reflection, and this appears to be the state of the eye ; for al- 
though we have two surfaces of the aqueous, two of the crys- 
talline, and two of the vitreous humour, yet we have only one 
reflected image, and that being from the anterior surface of the 
cornea, there can be no surface to reflect it back, and dilute 
an image on the retina. 
This hypothesis may be put to the test, whenever accident 
shall furnish us with a subject having the crystalline extracted 
from one eye, the other remaining perfect in its natural state; 
at the same time we may ascertain whether or no the crystal- 
line is that part of the organ, which serves for viewing objects 
at different distances distinctly. Seeing no reflection at the 
surface of the crystalline, might lead some persons to infer 
that its refractive power is very inconsiderable, but many cir- 
cumstances shew the contrary ; yet what it really is may be 
readily ascertained, by having the focal length and distance 
of a lens from the operated eye, that enables it to see objects 
the most distinctly ; also the focal length of a lens, and its dis- 
tance from the perfect eye that enables it to see objects at the 
same distance as the imperfect eye ; these data will be suffi- 
cient, whereby to calculate the refractive power of the crys- 
talline with considerable precision. 
Again, having the spherical aberration of the different hu- 
mours of the eye, and having ascertained the refractive power 
of the crystalline, we have data from whence to determine the 
proportional increase of its density as it approaches the central 
part, on a supposition that this property corrects the aber- 
ration. 
These observations of Mr. Ramsden respecting the use of the 
crystalline lens, I was very desirous of bringing to the proof ; 
