21,5 
Dr. Brewster on the laws of polarisation, &c. 
we have only to consider the position of our sphere within 
the crystal, and compute the inclination of the refracted ray 
to the axis. Thus in calcareous spar the axis P p corresponds 
with the short diagonal AB of the primitive rhomb, as shown 
in fig. 5 . In beryl, zircon, sapphire, rubellite , rock crystal, nephe- 
line, idocrase, muriate of lime, super acetate of copper and lime, 
7nuriate oj strontian, &c. it corresponds with the axis of the 
prism in which they generally crystallize ; while in hydrate 
of magnesia , arseniate of copper, &c. it is perpendicular to the 
laminae of which they are composed. 
Although the systems of rings produced by all crystals 
with one axis, exhibit the same tints, and possess the same 
properties, yet those which are produced by the five crystals 
of the negative class, namely, zircon, quartz, ice, super acetate of 
copper and lime, and certain specimens of sulphate of potash, will 
be found to differ in one essential particular from the system 
produced by all the other crystals. If we take two rhomboids 
of calcareous spar and place the one symmetrically on the 
other, the system of rings which they exhibit will be exactly 
the same as would have been produced by one rhomboid 
whose thickness is equal to the sum of the thicknesses of the 
two which are combined. M. Biot has deduced this result 
from theory, and attempted in vain to confirm it by experi- 
ment ;* but from the peculiarity of the method of observing 
* J’ai employe ainsi conjointement les deux plaques de spath d’Islande dont j’ai 
donne plus haut les epaisseurs ; et j’ai eu en effet des anneaux bien plus petits que 
par une seule d’entre elles j mais la difficulty d’ aligner egalement les axes de ces deux 
plaques m’a empeche de mettre dans les experiences l’exactitude necessaire pour les 
mesurer. J’aurais voulu aussi combiner des cristaux attractifs avec des cristaux 
repul.sifs, mais je n’en ai pas eu l’occasion. Biot Traill de Physique, tom. iv. 
