74 
ELEMENTARY CHEMICAL MICROSCOPY 
eyes open should see the field as a single bright circle. If two 
overlapping circles appear the oculars are too far apart. If 
the field is blurred and both eyes cannot simultaneously see the 
field, the oculars are too close together. A shutter which 
automatically remains open, operated by a lever, 5, is fitted 
below the prism chambers in order to assist in testing whether 
the proper pupillary distance has been secured. The observer 
looks into the instrument with both eyes, turns the lever 5 first 
to one side then to the other without moving the head. In this 
way it can be ascertained whether both eyes are in actual use. 
The shutter also serves in adjusting the focus of the paired objec¬ 
tives. In the higher powered objectives one of each pair is 
provided with a milled focusing collar m (on the right side). 
The worker places a suitable object on the stage and focuses 
the instrument; the shutter is then turned so as to cut off the 
view through the right half of the objective and the microscope 
is very carefully focused. The shutter is next turned so as to 
cut off the left half which has just been focused and if the image 
is not seen with equal clearness the focusing collar is turned 
until the image becomes clear and distinct. The instrument 
has now been focused for each eye and upon looking into the 
microscope with both eyes the object being studied should stand 
out stereoscopically and the image be clear and distinct. 
The mounting to which the two prism chambers are attached 
can be rotated in order that the worker may look into the instru¬ 
ment from the sides or front as the exigencies of the work may 
demand. This arrangement adds greatly to the value of the 
instrument. 
The magnifications available with this type of microscope 
lie between about 10 diameters and 150 diameters, with free 
working distances ranging from 70 mm. with the lowest power 
to 25 mm. with the highest power. This is more than ample 
to permit working with a variety of tools upon objects lying 
on the stage. Hand rests (removable) attached to the stage 
greatly facilitate manipulations. 
Two interchangeable stages are provided with each instru¬ 
ment, one of glass the other of metal; the opening in the metal 
