THE MICROSCOPE. 
part to be moved is attached. By turning this milled head one 
way and the other alternately, the observer finds by trial the 
position which gives greatest distinctness. 
33. This, which is called the coarse adjustment, answers 
well enough when high powers are not used; but it must be 
remembered that as the teeth of the pinion successively pass those 
of the rack, the motion produced is not strictly an even and 
uniform one, but a sort of starting or intermitting motion, so that 
the instrument cannot be easily and steadily brought to rest at 
any intermediate point between the beginning and the end of the 
passage of a tooth. When high powers are used, and conse¬ 
quently an extremely nice adjustment of the focus required, this 
arrangement is therefore insufficient, and serves at best only for a 
first approximation to the exact focus. 
34. A supplemental expedient is therefore provided in the 
best instruments, called the eine adjustment, which usually 
consists of a screw having an extremely fine thread, which 
being connected with the part to be moved, gives it a per¬ 
fectly smooth, uniform, and slow motion, entirely free from starts 
or jerks. 
In some of the best instruments these screws have as many as 
150 threads to the inch, so that one complete turn of the milled 
head moves the stage or body through only the 150th part of an 
inch, and as the head is divided into ten equal parts and moves 
under an index, a tenth of a revolution can be observed, which 
corresponds to the 1500th part of an inch. 
When the form of the object is not actually flat, and conse¬ 
quently all points upon it are not equally distant from the object- 
glass, they will not be all in focus together. When the distance 
of the object is such as to bring the more salient, and conse¬ 
quently the nearest, parts into focus, the more depressed parts 
will be too distant and consequently out of focus; and when the 
object is moved nearer to the object-glass by a space equal to the 
heights of the salient above the depressed parts, the latter will be 
in, and the former out of focus, and consequently the latter will 
be distinct, and the former confused. 
When the powers used are so low that the distance of the 
object from the object-piece shall bear a considerable proportion 
to the difference of level of the salient and depressed parts of the 
object, this difference of level will not sensibly affect the focal 
adjustment; but when high powers are used, that difference of 
level bearing a very sensible proportion to the distance of the 
object from the object-glass, the adjustment which renders either 
distinct will render the other indistinct. 
35. This optical fact has been converted with admirable address 
36 
