160 



Artificial Pupil, consisting of an anterior membranous and a poste- 

 rior muscular coat; the latter, in the act of contraction, presses 

 back the lens upon the vitreous humour, the elasticity of which 

 causes it again to advance when the sphincter muscle of the iris re- 

 laxes. In proof of the correctness of this opinion, Sir Everard ad- 

 duces some experiments illustrative of the influence of belladonna 

 upon myopic eyes. 



Having ascertained that the marsupium is not muscular, I could 

 only consider it, says the author, as a screen preventing the pencils 

 of rays that fall upon the portion of the retina within the axis of the 

 optic nerve, from extending to the outer portion. And after explain- 

 ing to Mr. Dollond, that the situation of the bird's eye in the head 

 makes the image of a distant object fall upon the retina within the 

 axis of the optic nerve, and of a near one without that axis, as the 

 bird only sees the object with one eye at a time ; but the human eye, 

 and that of quadrupeds, will have the image of a distant object fall 

 on the retina, without that axis, both eyes being turned to the ob- 

 ject; he said that the inner portion of the bottom of the bird's eye 

 was more extensive than the outer, and made a portion of a larger 

 curve ; consequently, was at a greater distance from the lens, and 

 therefore adapted to longer pencils of rays fitting it for distant vision ; 

 while the outer portion being nearer, the lens was fitted for seeing 

 near objects, the marsupium confining the rays, and preventing the 

 vision from being confused. 



In the human species and quadrupeds, the bottom of the eye has 

 one uniform curve, the portion within the axis of the nerve being 

 smaller than the outer, which is the very reverse of what it is found 

 to be in birds, adapting both eyes to see the same distant object at 

 one time. 



A Letter from John Pond, Esq. Astronomer Royal, to Sir Humphry 

 Davy, Bart. President of the Royal Society, relative to a Derange- 

 ment in the Mural Circle at the Royal Observatory. Read Novem- 

 ber 22, 1821. [Phil. Trans. 1822, p. 86.] 



On the Finite Extent of the Atmosphere. By William Hyde Wol- 

 laston, M.D. V.P.R.S. Read January 17, 1822. [Phil. Trans. 

 1822, ji 89.] 



It has been inferred from measuring the barometrical pressure at 

 different elevations, that the earth's atmosphere extends to the 

 height of 40 miles, beyond which limit we are left to conjectures, 

 founded on the supposed divisibility of matter ; which, if infinite, in- 

 dicates an infinity of atmosphere. If, however, it consist of ultimate 

 indivisible particles, then expansion of the medium composed of them 

 must cease at that point where the force of gravity downwards, upon 

 a single particle, is equal to the resistance arising from the repulsive 

 force of the medium. If we adopt the latter hypothesis, no part of 

 our atmosphere could ever leave the earth ; if the former, every pla- 



