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IX. An account of a Micrometer made of Rock Crystal. 
By G. DoLlond, F. R. S. 
Read January 25, 1821. 
Rock crystal having been applied to telescopes in various 
ways, for the purposes of micrometrical measurements, par- 
ticularly that which is recommended by M. Arago, induced 
me to consider if a more simple mode of applying the crystal 
could not be discovered ; and the following account of its 
application to the eye tube of a telescope, is the result. 
The improvement consists in making a sphere or lens from 
a piece of rock crystal, and adapting it to a telescope in the 
place of the usual eye-glass ; and from its natural double 
refracting property, rendering it useful as a micrometer. 
The advantages of thus applying the crystal are, in the 
first place, the very great saving of the time required to find 
the proper angle for cutting the crystal ; also of cutting the 
prisms to their proper angles, and working their surfaces 
with sufficient accuracy to render them useful as micrometers, 
in the manner that is recommended by M. Arago, Dr. 
Wollaston, and others. 
Upon the plan which is now submitted, it is only necessary 
to select a piece of perfect crystal ; and without any know- 
ledge of the angle that will give the greatest double refrac- 
tion, to form the sphere of a proper diameter for the focal 
length requir d. 
The second advantage is derived from being able to take the 
angle on each side zero, without reversing the eye tube; 
