[ IBl ] 



XXIX. On the Phcenomeim of Thin Plates of Solid and Fluid 

 Substances exposed to Polarized Light. Bij Sir David 

 Brewster, K.H., D.C.L., F.R.S., and V.P.R.S. Edin* 



[With a Plate.] 



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 instrument 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 outermost of which is black, 

 while the innei'most has various colours, or no colour at all, 

 in proportion to the quantity of vapour deposited. The co- 

 lours 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 

 circumferencesf. By using a large system of rings, or depo- 

 siting the vapour in straight lines in the plane of incidence, 

 we can at once observe the phaenomena of the coloured rings 

 or bands at various angles of incidence. 



The first person who investigated the modification of New- 

 ton'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 Memoires 

 d'ylrcueil, 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 phaenomena which 

 he observed were the same as those which had been previously 

 discovered by M. Arago, with the exception of the modifica- 

 tion 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 



* From the Philosophical Transactions, 1841, part 1. p. 43. 



t These rings may be formed upon almost all transparent bodies with 

 more or less brilliancy, though I have found several substance?, and occa- 

 sionally pieces of glass, that will not absorb the sosp. The rings are pro- 

 duced upon natural as well as artificial surfaces, that is, upon transparent 

 surfaces produced by fusion or crystallization, 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 llic same effect as soap. The rings disappear quickly by eva- 

 poration, and their brilliancy and purity of colour depend on the relative 

 temperature of the vapour and the glass. 



