ORIENTATING DEVICES 
141 
In order that full use may be made of a mechanical stage the 
amount of displacement must be indicated by equivalent scales 
on each of the two movements. It is therefore essential to find 
the value and the uniformity of the scale divisions and to find the 
diameter of the field of the microscope as indicated on the scale of 
the stage. This may be accomplished by laying a stage microm¬ 
eter in place between the clips F/ of the stage and measuring 
the displacement under a cross-haired eyepiece for different por¬ 
tions of each of the lateral scales of the stage. There is thus 
ascertained the true value of the graduations, whether both 
scales are equivalent and whether the scale divisions are of uni¬ 
form size. To determine the amount of stage displacement 
necessary to just include an entirely new area, bring a line of 
the stage micrometer just tangent to the circle of the field of 
view, read the stage and displace the micrometer until the same 
line is tangent to the field at the opposite end of the diameter 
of the field-circle and again read the stage scale. The difference 
in the readings will give the number of scale divisions necessary 
to bring an entirely new area of the preparation into the field 
of view with that particular optical combination which has been 
employed. 
When the entire area of the preparation must be studied, the 
student must of course look into the instrument while the prep¬ 
aration is slowly displaced in one direction, as, for example, to 
the right or left, and then turn the stage up or down the proper 
number of scale divisions and again observe the slowly changing 
field as it is displaced in the opposite direction to the left or 
right. 
Rotating or Orientating Devices. — It not infrequently hap¬ 
pens that irregular fragments of material must be carefully 
studied, or that the exact relation of one surface to that adjacent 
to it must be determined, or that the behavior of light rays sent 
through the body in different directions be ascertained. To 
facilitate the changing of the position of the substance and to 
enable the worker to so place it that the surface being examined 
shall lie in a plane normal to the optic axis, various orientating 
devices have been suggested. 
