SIR DAVID BREWSTER ON THE COLOURS OF MIXED PLATES. 77 



ceptibly alter the phase of the ray. When the plate, on the other hand, increases in 

 thickness, or the fluid diminishes in refractive power, the central bands will become 

 closer and more numerous, and will finally resemble the fringes within the shadow of 

 the ordinary system. 



When the plate of nacrite is thicker at one place than another by the partial re- 

 moval of a parallel film, the edge where the increase of thickness takes place produces 

 exactly the same phenomena as the edge of the film that is removed, or of the film 

 that is elevated above the general surface, and hence we are led to look for the phe- 

 nomena of mixed plates in minerals, such as sulphate of lime and mica, where a plate 

 of two different thicknesses can be easily obtained. I have accordingly discovered 

 the phenomena 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 repeatedly 

 in the gypsum from Mont-martre ; but they are most beautiful when the stratum has 

 a circular form. In this case the cavities are exceedingly 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 therefore also reversed, the highest order of colours being in the centre, 

 while the lowest are at the circumference of the circular stratum. In many strata of 

 cavities, such as the one which I have engraven in my paper on the new fluids in mi- 

 nerals*, 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 speci- 

 mens 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 com- 

 plementary 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 particles of transparent matter, or in which 

 the fluids have suffered induration. 



Allerly hy Melrose, 

 October ISth, 1837- 



* Edinburgh Transactions, vol. x. Plate II. fig. 33. 



