444 PHILOSOPHICAL TRANSACTIONS. [anNO ] 67 0. 



the vitreous humour, one shall not be able to see well through this membrane: 

 whence you conclude, that it is not transparent enough, to let so much light 

 as is sufficient for vision pass upon the choroides. 



In my opinion there is good reason to doubt of this consequence, as there 

 may be a great deal of difference between the retina of a dead animal, after it 

 has been exposed to the air, and that of a living animal, while it is exactly 

 shut up between the vitreous humour and the choroides. Different disposi- 

 tions ordinarily change the qualities of things ; fat, which is transparent when 

 melted, grows opaque when cold ; and the tunica cornea of an eye being held 

 some hours in one's hand, in a hot air, grows thick, and a little after entirely 

 opaque. But that you may be persuaded that the choroides is sufficiently 

 enlightened in a living animal, you must take the eye of an ox, newly killed, 

 while hot, and cut it in two in such manner that a good part of the vitreous 

 humour may remain extended on the retina ; then you shall see distinctly the 

 colours of the choroides, the basis of the optic nerve, the trunk of the little 

 vessels which proceed from thence, and their dispersion through the thickness 

 of the retina, with so much perspicuity, that you cannot even discern whether 

 there be a retina beyond the vitreous humour or not. Hence you may judge, 

 that the light which the objects send to the choroides is more than sufficient 

 for vision, seeing that being much weakened by the reflection, and by a second 

 passage through the retina and the vitreous humour, it is yet strong enough 

 to form a clear and distinct vision of the choroides in our eyes. 



Your second experiment to prove the opacity of the retina, which is to put it 

 into water, is also extremely deceitful. — For you make no doubt but the hya- 

 loides, which envelopes the vitreous humour, is perfectly transparent; ygt, if 

 you lay in a dish, half full of water, part of the vitreous humour, the parts of 

 the hyaloides which stick to it will appear whitish and thick, like a spider's web, 

 although the vitreous humour do still retain its transparency. It is not then a 

 sufficient proof to know whether the retina be opaque in a living animal, by 

 putting it into the water: and by what trial soever you make of it, after it has 

 been exposed to the air, you can draw no consequence to prove that it is opaque 

 in its natural state; for the crystalline itself becomes a little thickish in water, 

 and if it be left there some time, or exposed to the frost, it becomes white and 

 opaque like snow. 



It is therefore necessary for resolving our difference, and for knowing with 

 certainty whether the light of objects pass almost all entire to the choroides, 

 or whether it is almost all intercepted by the retina, to bring observations made 

 on the retina and the choroides, while they are in their natural state, as I shall 

 do in the following experiment. 



