268 On the Jffecllons of Light 



portions at a particular angle of incidence. The veins G^, Wh, 

 li, and Kk are, however, green when the surrounding portions 

 are red, and red when the surroundhig portions are green; from 

 which it follows that these veins produce a particular colour at 

 a different angle of incidence from the adjacent portions. la 

 another specimen of agate, very like the preceding, the same 

 phaenomena are distinctly visible, and the coloured image forme 

 the same angle with the common image. In a third specimen, 

 belonging to Rohert Ferguson, Esq. of Raith, the colours are 

 exhibited in the most splendid manner. A semi-transparent 

 and irregularlv elliptical zone, about six inches and a half in cir- 

 cumference and three-tenths of an inch broad, has i\\Q first va- 

 riety of structiu-e, and forms the coloured image at a distance 

 of 13^" from the conunon imaje. 



In the specimen represented in Plate IV., fig. 1, the colour* 

 are visible only in the vein AB ; but here the angle of the first 

 coloured image with the common image is 28°, while that of the 

 second image, which is very faint, is only a little greater. The 

 other vein CD, whidi to all appearance has the same structure 

 as AB, and which differs from it only in being a little thinner, 

 exhibits no colours ; but there is a small stripe st at its edge 

 ■where the colours are very distinct. This circumstance induced 

 TTie to tliink that the colours depended on the thickness of the 

 plate, as well as upon its structure; but upon grinding a hollow 

 place mvw in the vein AB, so as to make the agate remarkably 

 thin, I found that it gave the same colours as before. A similar 

 experiment was made with another piece of agate, and the re- 

 sult was the same, though the thickness of the plate could not 

 exceed the 400dth part of an inch. The colours indeed were 

 rendered more brilliant by the increased transparency of the 

 agate, but in other respects they exjjerienced no change. In 

 another specimen, of which it is unnecessary to give a particular 

 description, the coloured image formed an angle of about J^4^ 

 with the colourless pencil, and the different veins produced the 

 same colour at different angles of incidence. 



In attempting to explain these appearances, I at first ima» 

 gined that the colours arose from the polarization of the trans- 

 mitted ravs, and that they were analogous to the colours of 

 plates of mica and topaz whicli I have described in another place. 

 I found, however, from several experiments, that the coloured 

 image is equally distinct in every position of the agate ; that it 

 is alike produced by polarized or de])olarized light, and that it 

 suffers no change either when examined by a plate of agate or 

 by a doubly refracting crystal. 



The phaenomenon which we have described must therefore 



b* 



