342 
SECOND REPORT- 1832. 
Nordenskiold lias attempted a mathematical explanation of 
these colours, of which however I am not able to give any 
further account. Sir David Brewster has examined these 
curious phenomena; and it appears from his inquiries that the 
colours are produced on the principles of the colours of thin 
plates, by cavities bounded by parallel plane surfaces. 
In the application of polarized light to the examination of 
the properties of minerals and other crystals, we have acquired 
a new instrument, of a use far more extensive and instructive 
with regard to the structure and differences of substances, than 
anything which had before been dreamt of. Physical optics 
and crystallography are for the future two coordinate portions 
of a vast province of science, of which the limits as yet have 
not been caught sight of. In the optical examination of minerals 
there remains much to do ; and it would not be difficult to point 
out branches of inquiry which are of evident importance to 
the present state of our knowledge of crystals. It would, for 
instance, be very satisfactory to know* the difference of optical 
symmetry wffiich exists between a right and an oblique prism; 
—whether the additional deviation from geometrical symmetry, 
which occurs in the latter case, corresponds, in the optical 
properties, with the fact of our no longer finding the poles of 
the lemniscates in the same plane, as w r ould seem to be the 
case from some experiments of Sir J. Herschel,—or w hether 
this case is marked by some yet unguessed peculiarity. 
But wdiile we look forward with hope to the augmentation of 
the stores of observation in this most interesting department 
of the study of nature, it is desirable that we should be aware 
of the treasures already in existence. The discoveries already 
mentioned as published by Sir David Brewster, and many 
others which might have been added to the enumeration, form, 
I believe, but a part of the facts bearing on optical crystallo¬ 
graphy, which that indefatigable observer and acute philosopher 
has in his possession. He has long led the mineralogical w r orld 
to hope to receive from his hands a Treatise on Mineralogy, on 
optical principles, in w hich it may be presumed he will state 
all the most remarkable facts and law r s w ith regard to the rela¬ 
tions of mineral species to light, which have come under his 
notice; and certainly no acquisition could be more interesting 
to the mineralogist, or more likely to give a fresh impulse to 
the progress of this science *. 
The different optical properties of minerals have been theo¬ 
retically expressed by speaking of the different elasticity of the 
crystals in different directions. This induced Savart to examine, 
* I am sorry to learn from Sir David Brewster, that he does not contemplate 
the immediate publication of this long-desired work. 
