transmitted through crystallized Bodies. 191 
image, while the green rays compose the nebulous light, so 
that we have a red bright image enveloped in a cloud of green 
light. By turning round the agate 90® the bright image is 
formed by the green rays, while the nebulous image consists 
of the red rays, so that we have di green bright image encir- 
cled by a mass of jiebulous red light. If in the place of the 
agate we substitute a doubly refracting crystal, it will always 
be found that the ordinary image is green when the extraor- 
dinary one is red, and that they assume these colours alter- 
nately during the motion of the prism round the axis of vision. 
From these experiments, we may consider it as demon- 
strated, that the nebulous light has the same relation to the 
bright image, as the first has to the second image of all crys- 
tals that have the property of double refraction.* It does not 
appear, however, that the nebulous image is produced by a 
greater refractive power than that by which the bright image 
is formed. There is on the contrary every reason to conclude, 
in opposition to the analogy of all doubly refracting-f crystals, 
that the agate gives two images and polarises them like other 
crystallized bodies, while the one image is placed exactly in 
the centre of the other. 
II. On the Structure of the Agate as connected zvith its optical 
properties. 
When we examine a piece of transparent and well polished 
agate, we perceive a number of bands or stripes, which are 
* See Edinburgh Trans. Vol. VII. Part II. 
f It will be seen from a subsequent paper, that many other bodies both of mineral, 
animal, and vegetable origin have the property of forming two images polarised iu att 
opposite manner, but not produced by two different refractive powers. 
