336 
SECOND REPORT— 1832. 
pear to have noticed any differences between the double refrac¬ 
tion of crystals of different forms. Biot found grounds for 
separating doubly refracting crystals into two classes, charac¬ 
terized by opposite properties, which he called attractive and 
repulsive, or positive and negative; quartz is of the former, 
calc spar of the latter class. But no constant external distinc¬ 
tion of such substances could be detected : the only fact con¬ 
necting the form of crystals with their optical properties, was 
that observed by Haiiy,—that substances having the cube, oc¬ 
tahedron, dodecahedron, &c., for their primary form, had no 
double refraction. 
Sir David Brewster must be considered as in a great degree 
the creator of the science which studies the mutual dependence 
of optical properties and crystalline forms ; and he not only gave 
the first impulse to this study, but has enriched it with a vast 
quantity of most curious and interesting observations ;—so great 
indeed, that all which has been done by other labourers in this 
field, bears as yet no proportion to the amount of his contribu¬ 
tions. 
Some of Sir David Brewster’s first results* appeared, however, 
to contradict the general fact which we have just mentioned, as 
the only one then known on this subject, (. Edinb . Trans, viii. 
1815.) He found that some specimens of muriate of soda, fluor 
spar, and diamond, which according to the law just stated should 
have no optical axes, did, when they were obtained in consider¬ 
able thicknesses, exhibit the colours which had already been 
found to indicate double refraction. The crystals seemed to 
V 
consist of complementary parts, the effects of which nearly 
neutralized each other, but left in certain different parts of the 
crystal a small excess of action on one side and on the other. 
His next observations were on calc spar. He had already 
shown that the colours which appear in the specimens crossed 
by films are produced, not by these films as thin plates, but by 
the properties of polarized light; and he now found that these 
films have a crystallization of a position opposite to that of the 
rest of the crystal, as has already been stated in speaking of twin 
crystals. 
Another of Sir David Brewster s memoirs belono'ing to this 
O O 
period, is remarkably interesting. It had appeared by his 
* I do not dwell on the discovery,—one of the first announced by Sir David 
Brewster on such subjects,—that doubly refracting crystals have two dispersive 
powers corresponding to their two refracting powers; which discovery has recent¬ 
ly been re-stated as a novelty by Rudberg, and would have been again so re¬ 
stated by Mr. Cooper if he had not learnt from Sir David Brewster these previous 
publications of it. 
