sorby: optical characters of minerals. 
11 
give all three indices absolutely true, one at least may be cor- 
rect, and the others may be determined approximately. In any 
case the character and position of the images at once shows 
in what direction the section is cut, or the relation which any 
parallel planes of a natural crystal bear to the optic axes, 
though the phenomena are more complex than in the case of 
uniaxial minerals. 
It would occupy far more time than can be allowed on 
the present occasion to describe in detail the curious and 
anomalous appearance due to dichroism or to the laminar 
structure of particular minerals which gives rise to complex 
internal reflections. My chief aim has been to call attention 
to the very valuable facts which may be learned by viewing 
a circular hole or rectangular grating with a microscope 
through a parallel plate of any crystalline mineral. The data 
thus obtained are so remarkably characteristic that they alone 
would amply suffice to identify a large proportion of natural 
minerals. In many cases all the necessary observations can 
easily be made with small crystals in their natural state, 
which alone is of course a very great gain for practical 
mineralogy. The chief value of the method is, however, that 
it enables us to identify portions of minerals of microscopic 
size in sections of rocks as thin, or even thinner than 4-Jo-th 
of an inch, with an amount of certainty which leaves little to 
be desired. 
When examining specimens of such a size that their 
thickness must be measured by means of the scale attached to 
the body of the microscope, I find that an object-glass of 
about I inch focal length, combined with a somewhat highly 
magnifying eye-piece, gives the best results. When, however, 
we come to study the minerals in moderately thin sections of 
rocks, it is impossible to measure the thickness and the dis- 
placement of the focus sufficiently accurately by means of 
the scale and vernier. The fine adjustment screw of the 
