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V. On the Phenomena of Thin Plates of Solid and Fluid Substances exposed to Polar- 
ized Light. By Sir David Brewster, K.H. D.C.L. F.R.S. and V.P.R.S.Ed. 
Received April 15, — Read May 6 , 1841. 
HAVING received from Dr. Joseph Reade one of his beautiful instruments called 
the Iriscope, and made several experiments with it, I soon perceived that it might be 
advantageously employed in various investigations in physical optics. This instru- 
ment consists mainly of a plate of highly polished black glass, having its surface 
smeared with a solution of fine soap, and subsequently dried by rubbing it clean with 
a piece of chamois leather. If we breathe upon the glass surface, thus prepared, 
through a glass tube, the vapour is deposited in brilliant coloured rings, the outer- 
most of which is black, while the innermost has various colours, or no colour at all, 
in proportion to the quantity of vapour deposited. The colours in these rings, when 
seen by common light, correspond with Newton’s refected rings , or those which have 
black centres, the only difference being, that in the plate of vapour, which is thickest 
in the middle, the rings in the iriscope have black circumferences*. By using a large 
system of rings, or depositing the vapour in straight lines in the plane of incidence, 
we can at once observe the phenomena of the coloured rings or bands at various 
angles of incidence. 
The first person who investigated the modification of Newton’s rings in reference 
to polarized light was M. Arago, who has given an account of his observations in a 
beautiful and highly interesting memoir, in the third volume of the Mdmoires d'Arcueil, 
published in 1817- Without knowing what had been done by M. Arago, Professor 
Airy entered upon the same inquiry in 1831 and 1832; but the phenomena which 
he observed were the same as those which had been previously discovered by M. 
Arago, with the exception of the modification of the rings when formed by a lens 
pressed against the surface of a diamond. 
When Newton’s rings are formed by a lens pressed against a surface of glass, M. 
Arago observed that they were black centred, as usual ; and whether viewed with 
the eye or with a doubly refracting rhomb of Iceland spar, that the single or the 
* These rings may be formed upon almost all transparent bodies with more or less brilliancy, though I have 
found several substances, and occasionally pieces of glass, that will not absorb the soap. The rings are pro- 
duced upon natural as well as artificial surfaces, that is, upon transparent surfaces produced by fusion or cry- 
stallization, as well as upon those polished by art. The soap being gradually dissolved by the vapour, requires 
to be frequently renewed. I find that other substances, particularly some of the oils, produce the same effect 
as soap. The rings disappear quickly by evaporation, and their brilliancy and purity of colour depend on the 
relative temperature of the vapour and the glass. 
G 2 
