246 PHILOSOPHICAL TRANSACTIONS. [aNNO 1668. 



roides is found. For the trunks of the vessels of the retina are large and long 

 enough to extend on this or that side of the nerve, and so to hide some part of the 

 choroides according to their magnitude. And in this case it will be true that 

 vision is not made in all the parts where the choroides is found, though they be 

 exposed to the light. Which may very well give a check to your opinion, for- 

 asmuch as those trunks would hinder the objects falling on them from coming 

 to the choroides ; which would render the image deficient in that place, in re- 

 gard that those species would not be able to make an impression on the organ 

 of vision through those vessels. 



In the mean time such a discovery as this could not be long without being 

 confirmed. For as the secret of your experiment consists in contriving that the 

 picture of an object may fall just on the optic nerve, or thereabout, M. Picard 

 has devised a way by which an object is lost keeping both eyes open, by letting 

 the image of the object fall on both the optic nerves at one and the same time, 

 after this manner: 



Fasten against a wall a round white paper of the size of an inch or two, and 

 on the side of this paper put two marks, one on the right the other on the left 

 side, each about two feet distant; then place yourself directly before the paper, 

 at the distance of about nine feet, and put the end of your finger opposite both 

 eyes, so that it may hide from the right eye the left mark, and from the left 

 eye the right mark. If you remain firm in that posture, and look steadily with 

 both eyes on the end of your finger, the paper, which is not at all covered 

 thereby, will altogether disappear; which must be the more surprising, because 

 without this particular encounter of the optic nerves, where no vision is made, 

 the paper would appear double, the reason of which is sufficiently known. 



The application of this method is easily made to that of yours. For when 

 one looks steadily with both eyes on the end of one's finger, held before the 

 marks, it is the same thing as if you directed each eye by itself to the place 

 which is to be looked on to lose the paper; so that one may with both eyes do 

 the same thing, that you do with one, keeping the other closed. 



A Letter from Dr. Timothy Clarck, one of his Majesty s Physicians 

 in Ofdinary, concerning some Anatomical Discoveries and Observa- 

 tions, particularly the Origin of the Injection into Veins, the Trans- 

 fusion of Blood, and the Organs of Generation. iV° S5, p. 6*72. 



After some prefatory remarks on the state of medicine and physiology in his 

 days. Dr. Clarck animadverts upon the abuse of experimental philosophy; by 

 which some (he is persuaded) have acquired a temporary unmerited fame. 

 From this charge, however, he excepts, among anatomists, Asellius,* the dis- 



