sorby: optical characters of minerals. 
7 
bent so that the focal length is, as it were, increased by an 
amount depending on the thickness of the crystal and its 
refractive power. In order to see the lines in focus, it is 
therefore necessary to move back the body of the microscope. 
If we know the thickness (T) and the amount of the displace- 
ment of the focal length (cl), we can calculate the value of the 
T 
index of refraction (ft) from the equation, p = ^ 
These values are measured by means of a scale and vernier 
attached to the body of the microscope ; and, with care, there 
ought to be no error greater than 2 0 l 0 0 th of an inch. The 
thickness of the specimen (T) is determined by measuring 
the difference in the focal points for particles of dust on the 
surface of the supporting glass and on the upper surface of 
the mineral. In a similar manner the value of d is determined 
by the difference in the focal length for the lines of the 
grating seen through the supporting glass with or without 
the specimen under examination. From the value of the 
index thus determined a small amount must be deducted, 
depending upon the aperture and correction of the object- 
glass, and, when great accuracy is desired, several precautions 
must be taken to avoid a number of possible small errors, 
which it would be tedious to explain in detail. 
T 
In the equation, /x= j<_ c i it is assumed that the substance 
possesses no double refraction, as in the case of glass or 
crystals belonging to the regular system. When viewed 
through such a substance, only one simple and undistorted 
image of the circular hole can be seen, and both sets of lines 
are in focus at the same adjustment, no matter what may be 
their azimuth to the axes of the crystal. We obtain by 
measurements and calculation one single index of refraction, 
but this may vary so much in different minerals as to clearly 
point out what they are. Thus, for example, it varies from 
