Royal Society. 487 
was substituted for steam in these experiments. When moisture 
was present, the solid exhibited negative, and the stream of air po- 
sitive electricity ; but when the air was perfectly dry, no electricity 
of any kind was apparent. The author concludes with an account 
of some experiments in which dry powders of various kinds were 
placed in the current of air ; the results differed according to the 
nature of the substances employed, and other circumstances*. 
Feb. 9.—The following papers were read, viz.— 
1. “ Magnetical Term-Observations made at the Observatory at 
Prague, for September, October, November and December, 1842 :” 
by Professor Kreil. Communicated by S. Hunter Christie, Esq. 
Sec. R.S. 
2. “On the Structure and Mode of Action of the Iris :” by C. R. 
Hall, Esq. Communicated by P. M. Roget, M.D., Sec. R.S. 
After reciting the various discordant opinions entertained at dif- 
ferent periods by anatomists and physiologists, relative to the struc- 
ture and actions of the iris, the author proceeds to give an account 
of his microscopical examination of the texture of this part of the 
eye, in different animals. He considers the radiated plice, which 
are seen on the uvea in Mammalia, as not being muscular; but he 
agrees with Dr. Jacob in regarding them as being analogous in 
structure to the ciliary processes. The white lines and elevations 
apparent on the anterior surface of the human iris, he supposes to 
be formed by the ciliary nerves which interlace with one another in 
the form of a plexus. The iris, he states, is composed of two portions ; 
the first, consisting of a highly vascular tissue, connected by vessels 
with the choroid, ciliary processes, sclerotica and cornea, and abun 
dantly supplied with nerves, which, in the human iris, appear, in a 
front view, as thread-like striz ; and which are invested, on both 
surfaces, by the membrane of the aqueous humour. They are more 
or less thickly covered with pigment, which, by its varying colour, 
imparts to the iris on the anterior surface its characteristic hue ; and, 
by its darkness on the posterior surface, renders an otherwise semi- 
transparent structure perfectly opake. The second component por- 
tion of the iris consists of alayer of concentric muscular fibres, which 
fibres, in Man and Mammalia generally, are situated on the posterior 
surface of the pupillary portion of the iris; but which in Birds ex- 
tend much nearer to the ciliary margin, and consequently form a 
much broader layer. In Fishes and in some Reptiles they do not 
exist at all. 
The author then proceeds to inquire into the bearings which 
these conclusions may have on the physiology of the iris. He thinks 
that the phenomena of its motions can receive no satisfactory expla- 
nation on the hypothesis of erectility alone, or on that of the anta- 
gonism of two sets of muscular fibres; the one for dilating, the 
other for contracting the pupil. He is convinced that the contrac- 
tion of the pupil is the effect of muscular action; but does not con- 
sider the knowledge we at present possess is sufficient to enable us 
to determine the nature of the agent by which its dilatation is effected. 
* On the subject of Dr. Faraday’s paper sce the articles referred to at 
p- 5 of the present volume.—Epr1. 
