Sir D. Brewster on the Colours of Mixed Plates. 355 



Forbes Royle, M.D. ; Benjamin Travers, Esq. ; James Walker, Esq. ; 

 Charles Wheatstone, Esq. ; Rev. William Whewell, M.A. 



December 7, 1837. — No paper was read. 



Dec. 14, 1837. — The reading of a paper, entitled " On low Fogs 

 and stationary Clouds." By William Kelly, M.D. Communicated 

 by Captain Beaufort, R.N., F.R.S., &c., was resumed and con- 

 cluded.* 



The object of the present paper is to point out the circumstances 

 which influence the formation of low fogs, and to show what ana- 

 logy exists between the causes that produce them and those that 

 occasion certain forms of clouds, which may be considered as differ- 

 ing from fogs only in position. Having been attached for several 

 years to the naval party employed in the survey of the gulf and river 

 of St. Lawrence, the author had ample opportunities of observing the 

 phenomena in question. He concludes that the fogs described oc- 

 cur chiefly when the air is nearly saturated with moisture, and when 

 at the same time the temperature of the water on which they rest 

 either exceeds that of the air, or is considerably below it. These 

 fogs are generally very dense, often limiting the sphere of vision to 

 a few fathoms ; but seldom extend to any considerable height. 

 They do not often cover the land to any distance from the shore ; 

 and the tops of the hills, close to the water's edge, are clear, while 

 the bases, or sides, are enveloped in the mist. 



The following papers were then read ; — 



" On the Colours of Mixed Plates." By Sir David Brewster, 

 K.G.H., F.R.S., &c. 



In the prosecution of his optical inquiries, the author was induced 

 to study the phenomena of mixed plates, (originally discovered by 

 Dr. Young, and described by him in the Philosophical Transactions 

 for 1802,) as he had observed similar appearances in various mineral 

 bodies under analogous circumstances, to which he had been led to 

 ascribe an origin different from that assigned by Dr. Young. In 

 order to obtain a more distinct view of these colours. Sir David 

 Brewster employed, instead of the substances used by Dr. Young, 

 the white of an e.^^, beat up into froth, and pressed into a thin film 

 between plates of glass. From observations of the colours exhibited 

 by plates so prepared, and also by the edge of a thin film of nacrite 

 in contact with copaivi balsam, the author deduces the conclusion, 

 that all these phenomena, as well as those often seen in certain 

 specimens of mica through which titanium is disseminated, and also 

 in sulphate of lime, are cases of diffraction, where the light is ob- 

 structed by the edges of very thin transparent plates placed in a 

 medium of different refractive power. If the plate were opake, 

 the fringes produced would be of the same kind as those often 

 noticed, and which are explained on the principle of interference ; 

 but, owing to the transparency of the plate, fringes are produced 

 within its shadow ; and, owing to the thinness of the plate, the 

 light transmitted through it is retarded, and, interfering with the 



* We here resume our regular report of the Society's Proceedings, inter- 

 rupted in p. 206. 



2K2 



