by doubly refracting crystals. 37 
orange yellow tint along their axis, and a yellowish green 
tint in a direction perpendicular to the r axis. The exhibition 
of two colours in the same mineral by common light, was 
first seen in Iolite, and the Abbe Haiiy had the boldness to 
give it the name of Eichroite from this property, on the 
presumption that nature had limited it to this mineral. This 
dichroism , as it may be called, which, so far as I know, has 
never been observed in any other minerals than iolite and 
mica, is a very common property of crystallized bodies, as 
will be seen from the following section. 
Sect. II. On the absorption of polarised light by crystals with 
two axes of doable refraction. 
The general phenomena of absorption in crystals with two 
axes, are nearly the same as those which have been described 
in the preceding section ; but the quantity of light which the 
ordinary and extraordinary forces interchange, is regulated 
by new laws depending on the situation of the incident ray 
with respect to the two axes of double refraction. 
If O 0 and AB, a line perpendicular to it, are the two axes, 
and P,P', the resultant axes, or the poles of no-polarisation 
of Blue Topaz : then if COD, the 
plane of the resultant axes is per- 
pendicular to the plane of primitive 
polarisation, the polarised light in- 
cident on the plate at O will be blue 
after transmission. The blue tint 
preserves its intensity from O to A 
and B, the thickness of the plate 
being supposed to continue the same ; but as the incident ray 
MDCCCXIX. £) 
