transmitted through crystallixed Bodies^ 
1.97 
III. On the peculiar Colours exhibited by the Agate, 
In my former letter on the polarising 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 
polarised in a similar manner. I have since observed the same 
phenomenon in other specimens, and though I have not been 
able to discover its cause, I ti ust the following observations 
will be of some service to future inquirers. 
In the specimen represented, in Plate V., 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, H/, 1 ^, K^L, and when 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 colourless image of a candle will be visible, and on 
each side of it a lughly coloured image forming an angle of 
io°|- with the colourless image. The colours, which are ex- 
tremely brilliant, 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 distant from the eye, 
the colours appear diffused over the sur 'ace of the coarse 
grained portions, and when the light is strong, the phenomenon 
is uncommonly brilliant. When the vein B6 is a pale blue, 
at a certain distance from the eye, Qc and Dd are of the same 
colour, Et^ is greenisii, I7 is yellowish, FG^/is pale red, and 
the red colour is more intense towards L^. By a gentle mo- 
