Royal Society. 485 
varying in diameter from about the 20,000th to the 3500th part of 
an inch ; but measuring on an average about the 6000th part of an 
inch, and presenting a striking analogy with the dentine or ivory 
of the teeth. The last sections of the paper relate to the epidermis 
and the colouring matter of shells. 
References are made, in many parts of the paper, to illustrative 
drawings ; which, however, the author has not yet supplied. 
Jan. 26.—The following papers were read, viz.— 
1. “Observations on certain cases of Elliptic Polarization of 
Light by Reflection,” by the Rev. Baden Powell, M.A., F.R.S., 
Savilian Professor of Geometry in the University of Oxford. 
The author, by way of introduction, passes in review the la- 
bours of various inquirers on the subject of the elliptic polarization 
of light, and notices more particularly those of Sir David Brewster, 
who first discovered this curious property, as recorded in the Phi- 
losophical Transactions for 1830; of Mr. Airy, in the Cambridge 
Transactions for 1831 and 1832; and of Professor Lloyd, in the 
Philosophical Transactions for 1840, and in the Reports of the 
British Association for 1841. He then proceeds to give an account 
of his own experimental examination of the phenomena of elliptic 
polarization in the reflection of light from various surfaces, by ob- 
serving the modifications of the polarized rings under different con- 
ditions, both of surface and of incidence, and by endeavouring to 
ascertain both the existence and amount of ellipticity, as shown by 
the dislocation of those rings, and to determine its peculiar cha- 
racter, as indicated by the direction in which the dislocation takes 
place; the protrusion of the alternate quadrants appearing, in cer- 
tain cases, in one direction, and in others in the opposite. These 
observations are reducible to two classes; first, those designed to 
contribute to the inquiry, what substances possess the property of 
elliptic polarization, by examining the light reflected from various 
bodies ; and second, those made on certain cases of films of several 
kinds, including those formed on metals by oxidation or other action 
upon the metal itself, as well as by extraneous deposition. The 
author found the general result, in all these cases, to be, that from 
any one tint to another, through each entire order of tints, the form 
of the rings in the reflected light undergoes certain regular changes; 
passing from a dislocation in one direction to that in the opposite, 
through an intermediate point of no dislocation, or of plane polar- 
ization ; and thus exhibiting a dark and a bright centred system 
alternately, as long as the order of tints is preserved pure. ‘These 
changes in the form of the rings, he observes, are precisely those 
expressed by successive modifications of Mr. Airy’s formula, corre- 
sponding to the increments in the retardation which belong to the 
periodical colours of the films. 
The remaining portion of the paper is occupied by a description 
of the apparatus and mode of conducting the experiments; and of 
the observations made on mica, on decomposed glass, plumbago, 
daguerreotype, and other metallic plates, and on the coloured films 
produced on steel and on copper by the action of heat, and of voltaic 
