tranmilted through crystallized Bodies. 267 



their direction at the Hues ac, cm, de, the phaenomenon which 

 has been mentioned could not take place ; but whenever the 

 laminae change their direction, their inclination to tlie surface 

 Ukewise changes, and therefore the intensity of the transmitted 

 light experiences a corresponding variation as the rays have to tra- 

 verse different lengths of the imperfectly transparent laminae. 



When the veins and the laminae are incurvated like those in 

 the portion AftCi??, their inclination likewise changes; but as this 

 change is gradual and not sudden, as in the former case, the in- 

 tensity of the transmitted light suffers a gradual variation, and 

 the portion Aacm has the appearance of being concave. When 

 the laminae therefore are arranged in a circular form, they will 

 resemble a number of dimples, the apparent concavity of which 

 will in some cases depend on the curvature of the laminae, and 

 ■will exhibit the phaenomenon of the hammered agate. 



III. On the peculiar Colours exhibited ly the Agate. 



In my former letter on the polarizing power of the agate, I 

 noticed the existence of a coloured image which appeared on 

 each side of the common colourless image, and which was po- 

 larized in a similar manner. 1 have since observed the same 

 phaenomenon in other specimens ; and though I have not been 

 able to discover its cause, I trust the following observations will 

 be of some service to future inquirers. 



In the specimen represented in Plate IV., fig. 2, the colours 

 appear only when the rays of light are transmitted through the 

 veins B, C, D, E, F, G, H, I, K, or through the coarse grained 

 portions F^, Gh, \\i, Ik, KAL, andwhen these parts are covered 

 no colour is perceived. If the eye, therefore, is placed behind 

 any of the coarse grained parts, and close to the agate, a co- 

 lourless image of a candle will be visible, and on each side of it 

 a highly coloured image forming an angle of 101" with the co- 

 lourless image. The colours, which are extremely brilhant, are 

 blue, green, yellow, and red, reckoning from the common image. 

 A second image coloured in a similar manner, but considerably 

 fainter, is distinctly seen, forming with the colourless image an 

 angle of about 21°. When the agate is held some inches di- 

 stant from the eye, the colours appear diffused over the surface 

 of the coarse grained portions, and when the light is strong, 

 the phaenomenon is uncommonly brilliant. When the vein B6 

 is a pale blue, at a certain distance from the eye, Co and Dd are 

 of the same colour, Ee is greenish, F/is yellowish, FGq/is pale 

 red, and the vcd colour is more intense towards L^. By a 

 gentle motion of the agate the colours of these portions instantly 

 change, a particular colour being always produced in the same 



portion*- 



