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12 " Experiments on Light/' Bv Henry Fox Talbot, Esq., M.P., 

 F.R.S. 



In the first section of this paper, an account is given of certain ap- 

 pearances presented by transparent objects, and especially saline 

 crystals, viewed through a microscope, when illuminated by polarized 

 light. For this purpose, the first polarizing medium, consisting of the 

 arrangement of single-image calcareous spar, invented by Mr. Nichol, 

 is fixed beneath the stage of the microscope j and the second, which 

 is similar to it, is interposed between the eye-glass and the eye, and is 

 capable of being turned on its axis, so as either to allow of the trans- 

 mission of the whole of the light polarized by the first medium, or to 

 intercept the whole of it, according as its position is similar, or at right 

 angles to the former. In the latter case, any substance which has 

 the property of depolarizing the light transmitted to it by the first 

 medium, will appear luminous, while the rest of the field of view is 

 quite dark, and will exhibit the most brilliant colours, dependent on 

 its thickness and position ; so that if the stage of the microscope be 

 turned round, the colour of each crystal is seen to change, and gra- 

 dually to assume the complementary tint. Other variations in the 

 appearances are produced by interposing a plate of mica, which gives 

 a general tint to the whole field of view, and modifies the colours of 

 the objects viewed ; and also by turning the polarizing eve-piece, so 

 that the whole of the polarized light is transmitted ; when crystals, 

 which would be white if viewed by ordinary light, may be made to as- 

 sume various colours, and even sometimes to appear perfectly opaque ■ 

 a result which does not seem to be in accordance with that which 

 theory would lead us to expect. 



The second section is occupied with the development of a prin- 

 ciple which the author conceives is extensively applicable to the pur- 

 poses of photometry, or the accurate measurement of the intensity of 

 light. It is founded on the well-known experiment of the appearance 

 of a uniform grey tint presented by a circle, partly white and partly 

 black, when made to revolve rapidly; the intensity of the light being 

 regulated by the proportionvil spaces occupied by each colour. Se- 

 veral variations of this experiment are described, with a view to its 

 practical application to the proposed object. The author also suggests 

 the employment of methods founded on a similar principle to the 

 measurement of quantities in various other branches of physical 

 science ; for example, that of high temperatures. 



13. " On the Mummy Cloth of Egypt ; with Observations on the 

 Manufactures of the Ancients." By James Thomson, Esq., F.R.S. 

 Communicated by Dr. Roget, Sec. R.S. 



By subjecting the threads of various specimens of cloth, enveloping 

 Egyptian mummies, to accurate microscopic examination, which was 

 clone at the request of the author by Mr. Bauer, it was ascertained 

 that they were formed exclusively of the fibres of linen, and not of 

 cotton, a=i had been supposed • a conclusion which is corroborated by 

 other considerations stated by the author. The paper is accompanied 

 by drawings, exhibiting the appearances of the threads both of cotton 



