6 S0RBY : OPTICAL CHARACTERS OF MINERALS. 
we want is a method which will enable us to ascertain the 
approximate value of the principal optical constants, when 
we have at our disposal only detached, small, and imperfect 
crystals in their natural state, or those scattered about in thin 
sections of rocks, cut into plates which are inclined at every 
varying angle to the optic axes. 
The method now to be described satisfies these require- 
ments sufficiently well. Time would not permit me to give 
a full description of the apparatus, the manner of using it, 
and the conclusions to be drawn from the observed data. I 
must therefore avoid all unnecessary detail, and confine 
myself to such an outline as will serve to indicate the general 
character of the method. I may here say that I have received 
most valuable assistance from Prof. Stokes in the mathematical 
part of the subject. 
When, many years ago, I first commenced to apply the 
microscope to the study of minerals, I was told by a well- 
known professor of mineralogy that it would never be possible 
to learn much by the use of that instrument. If we merely mag- 
nify a portion of a pure transparent crystal, we do, indeed, learn 
little or nothing ; but if, instead of viewing the crystal itself, 
we look through it with a suitable magnifying power at some 
appropriate object, we can learn more facts of interest and 
importance than by any other single method whatever. The 
property possessed by the object-glass of collecting divergent 
rays to form an image gives rise to an entirely new class of 
phenomena, and converts the microscope into a most valuable 
apparatus for optical research. The object examined through 
the crystals is the image of a small circular hole, or of 
rectangular lines ruled on a piece of glass, formed at the focal 
point of a well- corrected achromatic condenser fixed below 
the stage, and so arranged that the image is placed either 
just below or just above the lower surface of the crystal. 
The divergent rays passing through it to the object-glass are 
