C 126 ] 
VII. On the methods of cutting rock crystal for micrometers. By 
William Hyde Wollaston, M. D. F R S 
v Read February 24, 1820. 
For the mere purpose of examining the phenomena of dou- 
ble refraction, ,t is extremely easy for any skilful workman 
to combine a wedge of rock crystal, or any other doubly re- 
acting substance, with another wedge of crown glass op- 
posed to it, in such a manner that a luminous object seen 
through them shall appear in its true place by ordinary re- 
fraction, accompanied by a second image at a s', nail distance 
produced by the extraordinary refraction of the crystal 
In consequence of the dispersion of colours which occurs 
m employing different substances, the above combination is 
not suu ed for the purpose of the micrometer invented by the 
Abbe Rochon ; but it is not difficult to obtain such a section 
of rock crystal as may be substituted for the wedge of glass 
so that the pencil of light shall be restored to its orfginal 
direction void of colour, without diminishing the separation 
of the images occasioned by the first wedge. 
But since the degree to which the double refraction of rock 
crystal separates the two portions of a beam of light trans- 
mitted through it, is not so great as may frequently be wished 
it becomes desirable to increase this effect beyond what can be’ 
produced by the most obvious method of employing that sub- 
stance ; and it does appear from M. Rochon's own accoum 
