264 On the Jff'ecHons of Light 



gives two images and polarizes them like other crystallized bodies, 

 while the one image is placed exactly iu the centre of the other. 



II. On the Structure of the Agate as connected with its optical 

 Proper ties. 

 When we examine a piece of transparent and well polished 

 agate, we perceive a number of bands or stripes, which are the 

 sections of a succession of laminae that are sometimes parallel, 

 but in general concentric. These lamina are often of a milky 

 white colour when seen by reflected light, and sometimes nearly 

 as transparent and colourless as glass, and the white laminas 

 commonly alternate with the transparent ones. The laminae 

 which are white when seen by reflected light, are brown by 

 transmitted light, and the intensity of this brown colour in- 

 creases with the thickness of the plate of agate. The transpa- 

 rent lan?inae exhibit three varieties of structure. 



The first variety, which appears to be the coarsest, consists 

 of a anmber of small serpentine lines like the figures J33333> 

 lying parallel to each other, and closely resembhng the surface 

 of standhig water when ruffled by a gentle breeze, or the sandy 

 bottom of a slow moving stream. These serpentine lines are 

 always arranged in a direction parallel to the laminas, and are 

 seen very distinctly even when the agate is so thin as the 150tl). 

 part of an inch. 



The second variety of structure differs from the first, only iu 

 the serpentine lines having a much smaller size ; and the laroinE& 

 which have this structure appear the finest and most transpa- 

 rent. 



The third variety has no serp-entlne linct-, and does not ap- 

 pear to differ from other semi-transparent bodies. It admits 

 the light more copiously in all directions than any of the other 

 structures ; and as it does not polarize it in a similar manner, 

 we may consider it as possessing, in a different way, that kind 

 of crystallization which polarizes the incident light by separating 

 it into two pencils. 



The white veins sometimes exlnbit the first variety of struc- 

 ture ; but in several specimens the veins appear to be fibrous in 

 their structure, tiie fibres stretcljing at right angles to the laminaa 

 through the whole of their thickness. 



These different stmctures will be better understood from fig. 1 

 and 2 of Plate IV. Fig. 1, represents the specimen of agate with 

 incurvated veins v/hich I have noticed in a former paper*. It 

 is composed of two veins, AB, CD, and three transparent por- 

 tions AEB, ABDC, and CFD. The transparent portions ex- 



* See Phil. Trans. 1813, part i.— riiil. Mag. vol. xlii, 



hibit 



