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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 F.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 reflected 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. 



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