sorby: optical characters of minerals. 
13 
In the case of the hard and brittle balsam used to fasten down 
the specimen, the value of m is about 1"54 ; but, if there be any 
doubt about the true index, it can be ascertained by special 
measurements. 
In a similar manner we may determine the index of some 
unknown mineral by comparing it directly with some other 
mineral lying near to it, the true index of which is either well 
known or has been previously ascertained from special 
measurements. For this purpose quartz is often very suitable, 
since its index varies very little. One great advantage of 
this method is that specimens may be observed far away from 
the edge of the section, provided of course that the minerals 
compared are so close together as to prevent any error due to 
unequal thickness in different parts. 
It must be borne in mind that, when any mineral has a 
very powerful double refraction, its apparent thickness, as 
seen through itself, varies according to the particular ray 
used for illumination and the direction of the objects chosen 
to determine the focal distances of the lower surface. There 
is, however, generally no difficulty in measuring with sufficient 
accuracy the mean apparent thickness, or that corresponding 
to some one image, and in calculating out the results 
accordingly. 
In connection with this subject it may be well to call 
attention to a somewhat interesting fact. If we have, side by 
side, two substances of different refractive power, but of the 
same absolute thickness, their apparent thicknesses, as seen 
through themselves, vary directly as the velocity with which 
light moves in them. Indeed, strictly speaking, the determi- 
nation of minerals in the manner now described depends 
entirely on an indirect measurement of the velocity with 
which light is propagated through them in different directions. 
In order to illustrate the practical applications of this 
method, I will describe the results obtained in the case of a 
