VOL. XIII.] PHILOSOPHICAL THANSXCTIONS. 645 



general considerations; as, that the impression of a luminous object remains 

 some time in the organ ; that some fibres being strongly moved, others near 

 them are also in motion ; that the eyes are always in motion, and very hard to 

 be fixed in one place, though it were desired. 



To the 2nd head he answers, that the concavity of the choroid cannot be 

 very rugged; for, on dissecting an eye and removing the retina, the surface 

 of the choroid has reflected an object as distinctly as a concave speculum. That 

 there appears no soil or dirtiness till the outward cuticle is broke, and then the 

 organ is disordered. As for the thickness of it, he says, he finds it in a man 

 but as a sheet of paper, or the pia mater in the brain. That the blood-vessels 

 are woven together with the nerves, on which account there may be as true a 

 sense of light in them as there is of pain in the hand when it is pricked with 

 the point of a needle; and perhaps the presence of veins and arteries in a mem- 

 ber is absolutely necessary to sensibility. 



To the 3d head, where M. Perrault gives reasons why there is no vision on 

 the base of the optic nerve, as first supposing that vision is to be made on a 

 smooth surface, the optic nerve, which is a bundle of fibres, is not smoothed 

 at its first entering the retina, but afterwards when the fibres are dissolved, and 

 spread into a coat, as when rags are made into paper. Here M. Mariotte, if I 

 rightly comprehend him, denies that the retina consists of fibres, affirming it to 

 have nothing but a mucousness, with some veins and arteries. But in an ex- 

 periment of Dr. Briggs's, a retina put into a glass of fair water, and drawn 

 about under water, both for the expanding and magnifying it, appeared plainly 

 to have a fibrous texture, like that of a piece of very fine lawn. In the 2nd 

 place, M. Perrault supposes that the choroid being pierced by the optic nerve, 

 there may come a light through the parts of the eye, the back way, into the 

 optic nerve, which would spoil the sense of another light coming through the 

 pupil. But this M. Mariotte will by no means agree to. 



Historta Naturalis Helvetice Curiosa. Authore Joh. Jacoho JVagnerOy M. D, 



Tiguri. N° 149, p. 268. 



The author professes that he undertook to write the natural history of Swit- 

 zerland on the invitation of my lord Bacon, and with an intention thereby to 

 promote a true experimental philosophy. The work is divided into 7 sections, 

 treating of the limits and nature of the soil ; of the Alps, the height, the ice, 

 snow, waters, &c. ; of the lakes, rivers, cataracts, &c. ; of the people, beasts, 

 birds, fishes, insects, serpents, trees, plants, fossils, metals, &c. 



