^46 Dr Brewster on the Phenomena qf' Dichrmstn 
another, not a single ray of light will reach the eye. The 
cause of this is obvious ; as the light transmitted through the 
first plate is all polarised in one plane, it is all absorbed by the 
second plate, when placed in a transverse position. See Phil, 
Trans. 1813. p. 103. Though this same fact is seen in Agate 
yet it becomes doubly interesting to observe the light all pola- 
rised in one plane, when the transmitted pencil is a maximum. 
3. On the Dichroism of lolite or Dichroite. 
This curious mineral, called lolite by Haiiy, from its bluish- 
violet colour, crystallises in six or twelve sided prisms, which 
appear of a deep blue colour^ when seen along the axis, and of a 
yellowish-brown colour^ when seen in a direction perpendicular 
to the axis, a b c d, Plate IX. Fig. 9. is a section of a 
prism of lolite, by a plane passing through the axis of the 
prism, the transmitted light will be blue through the faces b, 
and d c, and yellowish brown through a b c, and in every di- 
rection perpendicular to the axis of the prism. If we grind down 
the angles a, c, 5, t?, so as to replace them with faces m 
m' n\ and oy?, d p\ inclined 31° 41' to a J, or to the axis of the 
prism ; then if the plane abed passes through the resultant 
axes of double refraction, we shall observe, by transmitting po- 
larised light through the crystal in the directions « c, bd, and 
subsequently analysing it, a system of rings round each of these 
axes. The system will exhibit the individual rings very plain-^ 
ly if the crystal is thin ; but if it is thick, we shall observe, 
when the plane abed is perpendicular to the plane of primitive 
polarisation, some branches of blue and white light, diverging 
in the form of a cross from the centre of the system of rings, 
or the poles of no-polarisation, as shewn at p and p\ Fig. 10, 
where the shaded branches represent the blue ones. The sum- 
mits of the blue masses at p and p' are tipped with purple, and 
are separated by whitish light in some specimens, and yellow'-^ 
ish light in others. The white light becomes more blue from 
p and p' to o, where is it quite blue., and more yellow from 
p and p to c and d, where it is completely yellow. When 
the plane ab c d h in the plane of primitive polarisation, 
the poles p, p are marked by spots of white light, but every 
where else the light is a deep blue. 
Jn the plane cadb, Fig. 10. the mineral, when we look 
