Axes of Doubly Refracting Substances. By H. C. Sorby. 211 
being so cut that its longer axis is parallel to the principal axis of 
the crystal, we know that this longer axis is positive, and thus 
also at once know which is the positive and which the negative 
axis of the crystal under examination. We can also at once see 
what is the true order of colour which it gives, since we can 
readily count it up from the bands due to the quartz alone, seen 
crossing the field of the microscope. We are also by no means 
limited to visible tints. The crystal may have such a powerful 
double refraction or be so thick as to give apparently white light, 
and yet by using the thicker end of the quartz wedge the tints 
may be reduced down to those easily distinguished. 
It will thus be seen that by using this simple arrangement we 
secure all the advantages of a most unmanageably large number of 
plates of selenite, and can make all the necessary observations with 
ease and expedition. It now remains in conclusion to point out 
its practical use. 
In examining thin sections of rocks or loose material, crystals 
of elongated prismatic form are very often seen. The optical axes 
may or may not be parallel to the geometrical axis, and this is a 
character of considerable importance. Now, by means of the 
apparatus I have just described, we can at once ascertain whether 
it is the positive or negative axis which is more or less nearly 
parallel to the geometrical axis. This might serve to easily dis- 
tinguish two minerals. One might have its negative axis parallel 
to the prism, and thus might appear black when the quartz plate 
was parallel to it ; whereas in the case of the other mineral, the 
positive axis might be parallel to the prism, and it might appear 
black when the quartz plate was at right angles to the axis of the 
prism. Of course in making these observations we must be careful 
to arrange so that the plane of polarization of the light is at 45° to 
one of the axes of the crystal, and the axis of the quartz plate either 
parallel or at right angles to that axis as the case may require. 
Though I have hitherto specially alluded to crystals, the use of 
this method is by no means confined to true minerals. It may be 
employed with equal advantage in studying shells and other organic 
bodies which possess double refraction. We can thus at once see 
that in tbe shells of Pinna the negative axis is parallel to the axis of 
the prisms, as in the case of true crystals of calcite deposited on the 
wall of a vein or other surface of deposition, and in the case of some 
other shells may see that the negative axis of the aragonite is also 
perpendicular to the surface of their growth, as it also is when that 
mineral is deposited quite independent of any organic matter. 
