THE 
MONTHLY MICROSCOPICAL JOURNAL. 
NOVEMBER and DECEMBER, 1877. 
I . — On a New Arrangement for Distinguishing the Axes of 
Doubly Refracting Substances. 
By H. C. Sorby, F.R.S., President R.M.S. 
( Read before the Eoyal Microscopical Society, October 3, 1877.) 
In studying with the microscope thin portions of doubly refracting 
crystals, seen detached or in sections of rocks, very little attention 
has hitherto been paid to the direction of the positive and negative 
axes, although a knowledge of this may afford most valuable in- 
formation, as I have already shown in some of my former published 
papers. I must say that I am not at all surprised, since, though 
fully impressed with its importance. I have often neglected to make 
use of the form of apparatus hitherto employed, on account of the 
practical difficulties of the method and the uncertainty of the 
results. I have for a long time been anxious to devise some such 
modification of the necessary apparatus as would enable me with 
ease and certainty to ascertain which is the positive and which is 
the negative axis of any crystal, and have at length contrived a 
plan which appears to me in every respect satisfactory. 
In order that the method may be more clearly understood, it 
will be desirable to briefly describe a few well-known facts. If a 
thin section of a doubly refracting crystal cut obliquely or parallel 
to an optic axis be examined with polarized light and a crossed 
analyzer, it will in certain positions give colours by interference, 
the exact tints of which depend upon the thickness of the section 
and the intensity of its double refraction. If another section thus 
giving the same tint of the same order be placed over the other, the 
tints due to the combination depend on the manner in which they 
are placed in relation to one another. If their positive axes are 
parallel, the effect is the same as if the thickness were double, and 
the tints are raised ; whereas, if the positive axes are at right 
angles, and thus the positive axis of one parallel to the negative 
axis of the other, the doubly refracting power is, as it were, 
neutralized, and with crossed Nicols the field remains dark and 
the crystal looks black. If the crystal under examination vary 
somewhat in thickness so as to give by itself various colours, then 
VOL. XVIII. q 
