OPTICAL PHENOMENA OF ANCIENT DECOMPOSED GLASS. 201 



by the black- cross, are very remarkable. The following are some of the most in- 

 teresting cases which I have observed : — 



1 . The colour of the sectors is brownish near the intersection of the black 

 cross; then come two green bands or rings separated by a broad black band. 



2. White near the intersection, then a black band, and then a reddish yellow 

 one. 



3. The order of the bands from the centre or intersection of the black cross is 

 as follows: green, darkish line, yellow, black, brownish, and the outer band a 

 bright pure white. 



4. At the centre brownish red, then white, green, white, black, reddish, black, 

 faint light, black, brownish white. 



5. Olive green at the centre, a double red band, olive green, a broader red 

 band, then alternations of red, green, and white, the outermost band being a 

 bright white. 



These white, dark, and coloured bands, are obviously produced by the inter- 

 ference of pencils that have suffered one or more reflexions with the pencils 

 transmitted by the polarising films. 



6. In a remarkable specimen, from which I was able to detach many films, 

 the centre of the figure, or intersection of the black cross, was pale- white, par- 

 taking of the colour of the film. This was followed by distinct dark lines, indi- 

 cating the place where we begin to see through the edges of the hollow spheri- 

 cal surfaces. Beyond this line there were innumerable narrow rings, often of 

 brightly coloured light. This will be understood by examining Plate X. fig. 1, 

 and supposing that the centres of the decomposed cavities have been removed. 

 Through this central part there will be seen only the black cross. 



In various specimens of decomposed glass we find films which exhibit in 

 polarised light a number of bright and coloured lines often parallel to one 

 another, and so very small that they require a considerable magnifying power 

 to see them. They are shown, when large, in Plate X. fig. 5, where they 

 obviously are produced by a great number of cavities lying in a straight line. 

 Each cavity produces four luminous sectors, and as the middle of the cavity is 

 occupied with the black cross, the luminous edges of two opposite sectors form 

 the bright lines of polarised light. Upon turning the film round, these lines dis- 

 appear when they are in the plane of primitive polarisation, and have their 

 brightness a maximum when they are inclined 45° to that plane. When the 

 luminous lines are thus formed, we may generally satisfy ourselves by using high 

 magnifying powers, that they are produced by the edges of cavities ; but I have 

 observed bright lines and bands which must have a different origin, and which 

 I have no doubt arise from a structure existing in the glass previous to its 

 decomposition. 



In many of the fragments of ancient glass that have obviously been formed 



VOL. XXIII. PART II. 3 K 



