i88 Dr. Brewster on the Affections of Light 
discoveries; and it is probably from the cultivation of this 
department of physics, that philosophy will be enabled to un- 
fold the secrets of double refraction, to explain the forms and 
structure of crystallized bodies, and to develope the nature and 
properties of that ethereal matter, which, while it enlivens all 
nature by its presence, performs also a capital part in the 
operations of the material world. 
The diderent subjects of which I mean to treat in the fol- 
lowing letter may be included under five heads. 
I. On the polarising power of the agate. 
II. On the structure of the agate as connected with its op- 
tical properties. 
III. On the peculiar colours exhibited by the agate. 
IV. On the depolarisation of light. 
V. On the elliptical coloured rings produced by obliquely 
depolarising crystals. 
I. On the polarising Power of the Agate, 
I have already shewn, in a former paper, that a ray of light 
transmitted through a plate of agate cut by planes perpendi- 
cular to the laminae of which it is composed, suffers polarisa- 
tion like one of the pencils formed by double refraction. If 
the liglit thus polarised is incident at a particular angle upon 
any transparent body, so that the plane of reflection is per- 
pendicular to the laminse of the agate, it will experience a total 
refrcCtiv^n ; if it is transmitted through another plate of agate, 
having its laminse at right angles to those of the plate by 
which the light is polarised, it will sL.fBr total reflection; and 
if it is examined by a prism of Iceland crystal turned round in 
the hand of the observer, it will vanish and reappear in every 
quadrant of its circular motion. 
