REPORT ON MINERALOGY. 
337 
experiments, that compression and dilatation give to transparent 
bodies a structure which produces the same effects as the cry¬ 
stallization which is associated with double refraction. He now 
tried the effects of compression and dilatation on crystals ; and 
found that in this way the optical phenomena were variously af¬ 
fected, the rings deformed, the colours altered, the curves multi¬ 
plied, &c. And, as the law which regulates the influence of this 
mechanical tension on the previous crystalline tension of the sub ¬ 
stance, he found that “positive crystals, compressed so that the 
axis (or direction) of compression is parallel to the axis of the 
crystal, have the order of the tints raised .” The terms “ nega¬ 
tive,” “ dilated,” “ perpendicular,” and “ depressed,” of course 
alternate in the enunciation of this law, with “ positive,” “ com¬ 
pressed,” “ parallel,” and “ raised.” 
The striking and valuable generalization, however, which has 
established for ever a close connexion between crystallonomy 
and photonomy,—a connexion rich in the discoveries which it 
has already given us, and richer still in those of which it gives 
us no doubtful promise,—is found in the Phil. Trans, for 
1818, (the memoir having been sent in 1817). In this Sir 
David Brewster states that the extreme perplexity of the sub¬ 
ject, and the difficulty of procuring proper specimens, had pre¬ 
vented him hitherto from doing, what he has there done,— 
“reducing under a general principle all the complex appearances 
which result from the combined action of more than one axis of 
double refraction.” 
This law, so far as we are concerned with it as mineralogists, 
(for I am not now to speak of pure optical investigations,) is 
this :—That all crystals with one optical axis belong to the 
hexagonal or the pyramidal system (using the terms already ex¬ 
plained ; which are equivalent to those of Sir David Brewster,) 
-—that all crystals with three optical axes belong to the tessular 
system ;—and that all the crystals with two optical axes crystal¬ 
lize in other forms. It thus appeared that there was an exact 
correspondence between the degree and kind of the symmetry 
of the optical properties and of the crystalline forms. 
This important principle was not hastily snatched from a few 
observations, as men judging of great discoveries after the 
event, and struck by their simplicity, are always ready to think 
might have been done. It was carefully collected from an 
examination of many minerals, including 23 species with one 
axis, and 81 with two axes; and there were not wanting some 
apparent exceptions and difficulties in the application of the 
rule. These have for the most part disappeared under a closer 
examination. Perhaps one of the most striking instances of 
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