ii3 Sir George Shuckburgh's Account 
north and south. Note, this parallelism of the axis of the 
level, to the line of collimation in a direction east and west, 
does not appear to be a very important rectification, but on 
some occasions may have its use. 
(28.) Fifth. The refraction piece. After what has been 
done, this apparatus will be easily adjusted. Bring the tele- 
scope, by means of its two levels P k, and / m, to point to the 
horizon, and in the meridian ; then, by the two pinions f, h, 
(Tab. X. fig. 3) of the refraction piece, bring its two levels e 
and g to rest true ; move the nonius d, of the little semicircle 
of the horary and vertical angles a, b, c, to the middle of the 
divisions, or o° o', and also that of the little quadrant of alti- 
tude /, k, to o° o', and this part is adjusted. 
(29.) Sixth. The microscopes. The magnifying power 
and scale of the microscopes is all that remains to be con- 
sidered. 
The magnifying power of a compound microscope, as is 
well known, (see Smith's Optics, § 127) depends on the pro- 
portion between the distance of the object, and of its image, 
from the object glass, together with the proportion between 
the focus of the eye glass, and ordinary focus of the eye, 
looking at a small object (suppose of ~ or inch). These 
two ratios compounded give the power of the microscope. 
The former is called magnifying by distance, and is a ma- 
terial part in the construction of these microscopes ; the scale 
of the micrometer being regulated by this part of the magni- 
fying power. For example, let the distance of the object from 
the glass be == 1, and the distance of its image = 4, its power 
will be 4 ; and consequently the scale of the micrometer, or 
motion of its screw, to ^answer to io', (suppose) must be 
