micrometer made of rock crystal. 103 
holes, that when its natural axis is parallel to the axis of the 
telescope, only one image of the object is seen. In the 
other direction, or that which is at right angles to the axis of 
motion, it must be so placed, that when it is moved, the 
separation of the images, viz. the ordinary and extraordinary, 
may be parallel to that motion. The method of acquiring 
this adjustment is, by turning the sphere in the half holes 
parallel to its own axis. 
The field of view of the eye tube is increased, and the 
magnifying power varied, by the introduction of the lens d, 
fig. 1, between the sphere and the primary image of the ob- 
ject-glass ; and its distance from the sphere will be in pro- 
portion to the magnifying power required ; the magnifying 
powers are engraved upon the eye tube at e, fig. 2, and will 
vary in proportion to the focal length of the object-glass 
to which the eye tube is applied. 
Those marked in the figure, are for an object-glass of 4^ 
inches in focal length. 
When I constructed this micrometer, it was my intention 
to have applied it to the measurement of the angles that are 
subtended by the apparent diameters of the fixed stars, as 
seen in achromatic refracting telescopes, for the purpose of 
determining their relative magnitudes ; also of measuring 
the distances of those double stars that would come within 
the range of the micrometer ; but from being compelled to 
attend to business of more immediate consequence, I am not 
able to accompany this description with any measurements 
that are sufficiently important to be interesting ; although I 
am fully convinced from the trials I have made, that the 
micrometer is quite equal to the purposes for which it was 
intended. 
