200 SIE DAVID BEEWSTER ON THE STRUCTURE AND 



balsam rushing out in consequence of being displaced by the alcohol. If we allow 

 part of the balsam to remain in (A), its colour will reappear, and be brighter and 

 deeper than that of (B), or its own original colour. 



When the surface of any film is specular, or slightly roughened by innumer- 

 able cavities, which the microscope is required to discover, or when it is covered 

 with flat polygonal cavities, which rise or sink but slightly beneath the general 

 level, and which are separated by a reticulated structure, namely, the outlines of 

 the polygon, the colour of the plate is uniform, or of one tint ; but when the 

 cavities are deep, and especially when they are not very numerous, their colour is 

 not the same as that of the rest of the film. When of the same colour, it is some- 

 times less and sometimes more intense. In many specimens the deep cavities 

 have a different colour from the rest of the film, as in Plate X. fig. 4, and Plate 

 XI. figs. 6, 7, and 8, the middle or apex of the cavity having often a paler ring 

 round it. Sometimes one side of the cavity has a different colour from the rest 

 of it, which may arise either from its elementary films being more inclined to 

 the incident light, or more numerous. 



In a few specimens where the colour is uniform, and consequently the thick- 

 ness of the film equal, I have observed bands of a different colour, which it is dif- 

 ficult to explain, as the microscope does not show any structure connected with 

 the bands. In some specimens the film is crossed with differently coloured bands 

 and lines extremely narrow and fine, but as they are bounded by sharp edges, 

 they must arise from parallel veins existing in the glass previous to its decompo- 

 sition. Other films exhibit differently coloured bands, which arise from lines of 

 minute cavities, which, as we shall see, is proved by their action upon polarised 

 light. 



In a paper " On the Action of Uncrystallised Films upon Common and Polarised 

 Light,"* I have described the phenomena exhibited by transparent films of 

 decomposed glass, when exposed to these two kinds of light. In polarised light 

 the deep cavities in colourless plates, whether polygonal, spherical, or oval, 

 exhibit distinctly, though with different degrees of brightness, the black cross, 

 with the four luminous sectors of the system of rings seen along the axis of uni- 

 axial crystals. The tints in these sectors never rise above the white of the first 

 order. 



When the films are coloured, the luminous sectors have frequently the same 

 colour as the film, but in many cases they are different. In a film orange red by 

 common transmitted light, the colour of the luminous sectors shown in polarised 

 light is Uue. In another, blue in common light, the luminous sectors are red, as in 

 Plate X. fig. 4. In a third, with green in the centre of the cavities and i^ed round 

 the margin of the cavity in common light, the colour of the luminous sectors is pale 

 red. In many other specimens the rings, or circular bands, in cavities traversed 



* Edinburgh Transactions, vol. xxii. p. 607- 



I 



