like manner as (lie hath fpread over our skin an infenfible Epi- 
dermis, to keep it from being too eafiiy hurt by thofe bodies 
that touch us, and by the Excefs of heat and cold. But if I 
fliould abfolutely deny, that the Retina hath any opacity in a li- 
ving Animal, your Experiment would not convince me, it be- 
ing made but upon a Retina^ whofe more fubde and tranfpa- 
rent parts are evaporated 5 and I might propofeto you for an 
example a piece of white paper, through which^ when it is 
wet, one fees diftinftly enough whatever is contiguous to it| 
and which prefently refumes its firft opacity, when it js a little 
while^Kpofed to the Air: And if this Example were not fujfici- 
ent, I might ailedge t^ e little Chryftalline, which is found in 
the middle of the Chryftalline humour of many Animals, and is 
as it were the kernel of it; the which, being as tranfparent as 
the other humours of the Eye in a living Animal^becomes m two 
or three dayes after his death white and opacous , notwith- 
flanding that it be left inclofed in the Eye, and that the Exte- 
riour Chryftalline remain ftill tranfparent. 
Your /^r^?;;^ Experiment to prove the opacity of the Retina^ 
which is to put it into water, isalfo extremely deceitful: For, 
you make no doubt but the Hjaloeides^ which invelopes the Vi- 
treous humour, is perfectly tranfparent: neverthelefs if you lay 
inadifh half full of water, part of the Vitreous humoor-> the parts 
of the Hialoeides, which ftick to it , will appear whitifli and 
thick like a fpiders web, although the Vitreous humour do ftill 
retain its tranfparency. It is not then a fofficient proof to know, 
whether the Retina be opaque In a living Animal, by putting it 
into the water : And by what tryal foever you make of it, after it 
has been expofed to the Ay re, you can draw no confequence to 
prove that it is opaque in its Natural ftate: For the Chryftalline 
it ielf becomes a little thickifli in water^ and if it be left there 
fome time, or expofed to the froft, it becomes white and opaque 
likefnow. 
It is therefore neceffary for refolving our difference, and for 
knowing with certainty, whether the light of objeds pafs al- 
llf^inc ft all t ntire to the choreeides^ or whether it is almoft all inter- 
' cepted by iht Retina^ to bring obfervations made on the Retina 
H 2 and 
