THE MICROSCOPE. 
minute upon the glass, with the necessary precision, especially 
when it is remembered that any error or inequality will necessarily 
he augmented in the exact proportion of the magnifying power 
with which such a scale is seen. Nevertheless this difficulty has 
been most successfully overcome, and combinations of screws and 
Fig. 24. 
wheels have been contrived, by which the diamond point is moved 
by self-acting mechanism, so as to trace the successive divisions 
of scales of astonishing minuteness. Scales are thus produced, 
the divisions of which are no greater than the 25000th part of 
an inch. 
This extreme minuteness is, however, rarely necessary or desira¬ 
ble in microscopic researches, and the divisions of the scales in 
more common use vary from the 1000th to the 2000th of an inch. 
In the scales delivered with moderately good French instruments, a 
millimetre is divided into one hundred parts. A millimetre being 
about the 25th of an inch, these divisions would therefore be the 
2500th of an inch. (See Tract on Microscopic Drawing and En¬ 
graving, Museum, vol. vi.) 
The process described above, in which the object is measured 
by superposition upon the micrometric scale, is attended with 
several practical difficulties and objections. The object, when 
thus placed, is always nearer to the object-glass than the scale, 
and when it is in focus, the scale is out of focus and invisible; 
and, on the other hand, when the scale is in focus, the object is 
out of focus and indistinct. When low powers only are used, 
this difference between the focus of the object and that of the 
scale being inconsiderable, will not prevent the success of the 
operation ; but when the powers are high, it can never be satis¬ 
factorily, and sometimes not at all effected. 
There is still another objection to the process. The placing and 
displacing of objects frequently on a surface so delicately engraved, 
subjects it to friction, which soon spoils and effaces the divisions. 
If the divided surface be protected, as it may be, by a plate of 
glass laid upon it, the difference between the distances of the 
object and the scale from the object-glass is augmented by the 
52 
