204 SIR DAVID BREWSTER ON ANCIENT DECOMPOSED GLASS. 



exhibited, had there been no small cavities in it, is not shown even in the imperfect state 



in which it exists, owing to its eclipse by the smaller cavities. 

 Fig. 5. Is a film seen by polarised light, and exhibiting the cavities arranged in lines, as described 



in p. 193. The drawing of this film as seen in common light, was accidentally omitted 



by the artist. It had the same colour as fig. 12. 

 Fg. 6. Shows the right-hand part of fig. 4 in polarised light. (See fig. 4.) 

 Fig. 7- Is another specimen as shown by polarised light, having an elliptical cavity exhibiting a 



difiused black cross. 

 Fig. 8. Show groups of crystals of silex formed between the films, in transmitted light. 

 Figs. 9 and 10 show the same magnified. There are numerous examples of the leaf-like branches 



shown in fig. 9, and still more numerous examples of the silex occurring in circular 



crystals of singular beauty and various forms. (See p. 194.) 

 Fig. 11. Shows a film as seen by reflected light, the convex sides of the cavities being towards the eye. 



(See p. 188.) 

 Fig. 12. Shows, very imperfectly, in the faint blue lines, the fi*inges produced by the different thick- 

 nesses of the plate or plates of air between the films of glass. See p. 191. 

 Fig. 13. Shows a film as seen by reflected light, the concave sides of the cavities being towards the 



eye. (See p. 188.) 

 It is often difficult to determine whether the convex or the concave sides of the cavities are 

 towards the eye, when they are seen by reflected light ; but the image of the flame of a candle in the 

 inverting microscope is always on the 7-ight hand of the cavity when its concave side is towards the 

 eye, and vice versa. 



N.B. — I have used the word cavity to denote each spherical group of films, like a number of 

 watch-glasses placed within one another, the concave side or cavity being supposed to be towards 

 the eye — the optical phenomena seen by transmitted light, whether common or polarised, being the 

 same when the convex sides are towards the eye. 



