REPORT ON MINERALOGY. 
341 
no double refraction at all. This remarkable circumstance was 
confirmed by the most decisive experiments, and now offers 
no difficulty when viewed in connexion with the undulatory 
theory. 
A somewhat similar circumstance has been discovered by Sir 
David Brewster in some specimens of glauberite. They are 
biaxal for red rays, the resulting axes being 5° asunder; but 
the axes for violet rays coincide, and for such light the crystal 
is uniaxal. This remarkable peculiarity was detected by the 
use of homogeneous light. 
We have still another fact to notice equally striking, equally 
unexpected, and having also the name of Herschel associated 
with its discovery. There existed an optical law which had 
already attracted the attention of philosophers as being entirely 
anomalous and sui generis , and a crystallographical peculiarity 
equally curious. Sir J. Herschel, with singular sagacity and 
felicity, showed that these two circumstances were constantly 
conjoined. I speak of the circular polarization of light to the 
right or left, and the plagihedral crystallization of quartz. In 
both these cases we had, instead of the geometrical symmetry 
by which the laws of nature are usually marked, a set of ap¬ 
pearances suggesting the idea of progress round a circle to the 
right or left hand ; the deviation of the plane of polarization, as 
shown by the succession of colours on increasing the thickness 
of the transparent plate, being the optical fact thus governed, 
and the oblique position of certain faces of the crystal the mi- 
neralogical fact. It was proved that right-handed polarization 
always accompanies right-handed plagihedral faces, and left- 
handed polarization left-handed faces. This was established 
by Sir J. Herschel from the examination of thirteen crystals, 
and has since been fully confirmed by other observers. 
It does not properly belong to our subject to dwell upon 
Prof. Airy’s theory of the circular and elliptical polarization 
of the rays of light in quartz, by which an extremely complex 
and apparently unsymmetrical collection of phenomena are re¬ 
duced to the most complete simplicity and regularity. But we 
may mention the experiments of the same observer on the 
optical properties of diamond. It appeared from these, that 
diamond, instead of completely polarizing light reflected at a 
certain angle, as other transparent substances are found to do, 
presents, at the angle at which light is most nearly polarized, 
phenomena resembling rather those of a surface of metal, than 
of a diaphanous medium. 
Among optical inquirers, several persons have employed 
themselves in researches on the causes of the play of colour 
which is seen in Labrador felspar, as Hessel and Genf; and 
