196 Mr. Talbot's Account of the 



are led to look for the phsenomena of mixed plates in mine- 

 rals, such as sulphate of lime and mica, where a plate of two 

 different thicknesses can be easily obtained. I have accord- 

 ingly discovered the phcenomena of mixed plates distinctly 

 exhibited in sulphate of lime and mica. 



A more splendid exhibition of these colours is seen when a 

 stratum of cavities of extreme thinness occurs in sulphate of 

 lime. I have observed such strata i-epeatedly in the gypsum 

 from Mont-martre ; but they are most beautiful when the stra- 

 tum has a circular form. In this case the cavities are exceed- 

 ingly thin at the circumference of the circle, and gradually 

 increase in depth towards the centre, so that we have a series 

 of edges increasing in thickness towards a centre ; the very 

 reverse of a mixed plate, such as a film of albumen pressed 

 between two convex surfaces. The system of rings is there- 

 fore also reversed, the highest order of colours being in the 

 centre, while the lowest are at the circumference of the circu- 

 lar stratum. In many strata of cavities, such as the one which 

 I have engraven in my paper on the new fluids in minerals*, 

 the cavities are too deep to give the colours of mixed plates. 



Another example of the colours of mixed plates in natural 

 bodies occurs in specimens of mica, through which titanium 

 is disseminated in beautiful flat dendritic crystals of various 

 degrees of opacity and transparency. In these specimens the 

 titanium is often disseminated in grains, forming an irregular 

 surface. The edges of these grains, by retarding the light 

 which they transmit, produce the direct and complementary 

 colours of mixed plates in the most perfect manner, the tints 

 passing through two orders of colours as the grains of titanium 

 increase in size towards the interior of the irregular patch. I 

 have observed another example of these colours in the deep 

 cavities of topaz, from which the fluids have either escaped, 

 leaving one or both of the surfaces covered with minute parti- 

 cles of transparent matter, or in which the fluids have suffered 

 induration. 



Allerly by Melrose, Oct. 18, 1837. 



XXXVII. Some Account of the Art of Photogenic Draxving. 

 By H. F. Talbot, Esq., F.R.S4 



§1- 

 TN the spring of 1834 I began to put in practice a method 

 -■■ which I had devised some time previously, for employing 



• Edinburgh Transactions, vol. x. plate ii- fig. 33. 

 t Read before tlie Royal Society on the 31st of January, and communi- 

 cated by the Author. 



