COARSE AND FINE ADJUSTMENTS. 
into an expedient, by which the inequalities of the surface of a 
microscopic object are gauged, and its accidents analysed. Thus, 
for example, let the milled head of the tine adjustment be first 
turned so as to render the salient parts distinct, and let the 
position of the index be marked. Let it be then turned so as to 
render the depressed parts distinct, and let the new position of 
the index be marked. If one division of the head represent the 
1500th part of an inch, the differences of level, of the salient 
and oppressed parts, will be just so many 1500ths of an inch as 
there are divisions of the milled head which have passed the 
index. 
36. One of the first difficulties which the microscopic debutant 
encounters, is that which will attend his attempts to bring the 
object into the centre of the field of view when it is minute, and 
when the magnifying power is considerable. If he is only pro¬ 
vided with a simple stage, without any mechanical expedient for 
moving the object, he will soon be oppressed with the fatigue 
arising from a succession of abortive attempts at accomplishing 
his purpose. 
37. The enjtire diameter of the field of view will often be less 
than the 100th. of an inch, so that a displacement of the slide so 
inconsiderable as to be utterly insensible to his fingers, will cause 
the object to jerk through a space greatly exceeding the entire 
extent of the field. In this way the object will start from side 
to side, the motion imparted to it by the touch to bring it back 
to the field being always in excess, however carefully and deli¬ 
cately the manipulation may be made. Some professional 
observers, by intense and long-continued practice, surmount this 
difficulty and succeed in adjustiug the slides, even with the 
highest powers, without mechanical aid; but this is not to be 
hoped for by debutants or amateurs, except with very low magni¬ 
fying powers. Such persons, if they would avoid the risk of 
throwing up the instrument with disgust, had therefore better in 
all cases be provided with a stage having some such expedients as 
we shall now describe. 
Upon the fixed stage, such as it has been described, a second 
stage similar in form and equal in size is placed, and is moveable 
through a certain limited space right and left, by a fine screw 
with a milled head. Another similar stage is placed upon this, 
which partakes of any motion imparted to the latter, but which 
is also moveable upon the latter backwards and forwards by means 
of another fine screw. Upon this last stage the slide with the 
object is placed, and held down by springs so as to retain its 
place, whatever be the position of the stage. 
By turning one of these screws (fig. 19), the object may be 
37 
