The President's Address, E. C. Sorhy, F.B.S., P.G.S. 15 
found in lava, no doubt owing to the presence of a larger amount 
of iron. 
It will thus be seen that the study of the indices of refraction 
in the manner I have described enables us not only to identify with 
more confidence each particular mineral, by bringing to bear a 
most important class of optical characters, hitherto unavailable in 
studying thin sections of rocks, but also in some cases enables us 
to form a very satisfactory opinion respecting certain variations in 
chemical composition. Though all this is possible, yet it most 
certainly requires a far more strict attention to minute details than 
is ever taken into account in ordinary microscopical research. The 
accurate measurement of the up and down movement of an object- 
glass to within ygiou is a very different matter to 
measuring any such visible quantity with an ordinary micrometer, 
but then we must remember that by this step we convert the 
microscope into an all but new physical instrument. However, if 
some microscopists might not feel disposed to attend to all the 
minute detail necessary for accurate quantitative observations, the 
methods now described may easily be employed qualitatively, and 
many valuable conclusions drawn from what may be seen without 
any actual measurements. The difference in focal distance and 
the focal character of the images are easily observed, and much 
may also be learned from the manner in which the images are 
separated. This alone may prove that a minute crystal, seen under 
the microscope, has two optic axes inclined to one another at a 
greater or less angle. Since this is a fact of some interest, and 
has I believe not yet been applied to microscopical research, it will 
be well to notice it in some detail. 
As previously named, if a parallel plate of any doubly refracting 
mineral be cut in the plane of any two of the focal axes, the two 
images, polarized in opposite planes, are not separated horizontally. 
When cut obliquely, the results vary with the direction of the 
section and the character of the crystal. If it have one optic axis, 
and therefore only one focal axis, the line along which the images 
are separated is in all cases parallel to this axis, and therefore 
parallel to the plane of polarization of one of the images. If it 
have two optic axes, and therefore three focal axes, and the section 
be cut obliquely to two of them, the images will be separated in 
relation to both of them, and the resultant line of maximum 
separation will not be parallel to the plane of polarization of either 
image. These facts will be better understood by means of Figs. 9, 
10, and 11. 
In both cases the planes of polarization of the two images are 
supposed to be parallel to one or other of the two systems of per- 
pendicular lines of the grating (Fig. 9). This is easily arranged 
practically. The plane of polarization of a polarizer under the 
