190 
ELEMENTARY CHEMICAL MICROSCOPY 
coordinate paper. This curve can then be employed in future 
measurements. It is obvious that the nearer the scale is to the 
microscope the greater will be the magnitude of the scale image, 
and the farther the scale the smaller the graduations will appear. 
Once the “ curve ” is obtained, we have at our command a 
device for accurate measurements (for all save very minute 
objects), by means of a scale the working magnitude of whose 
divisions is varia,ble at will between wide limits. 
This method of micrometry is especially convenient when 
employing binocular microscopes or where special rulings are 
required in quantitative work. The use of specially ruled glass 
cells is thus avoided.^ 
Method 6. — Micrometry by means of the fine adjustment 
micrometer screw. Most microscopes are provided with a fine 
adjustment so constructed with micrometer screw, accurately 
ground wedge or cone as to permit measurements of the thick¬ 
ness of objects through a determination of the amount of dis¬ 
placement necessary to focus the instrument upon the lower 
and the upper surface of the object. The amount of displace¬ 
ment is indicated by a graduated head or drum attached to the 
fine adjustment moving past a fixed index. 
The value of one scale division of the drum is usually marked 
by the maker upon the instrument or indicated upon the table 
of magnifications accompanying the microscope when purchased. 
If this value is unknown it may be ascertained by placing an 
object of known thickness having parallel sides upon an object 
slide, clamping as tightly as possible to the slide with the stage 
clips and focusing first upon the slide, then upon the upper sur¬ 
face of the object. The difference in the fine adjustment drum 
readings will give the number of divisions equivalent to the 
thickness of the object. The thickness of the object used may be 
determined by placing it edgewise on the stage and measuring 
its thickness by any one of the micrometric methods given above. 
1 Dr. W. W. Andrews of Regina, Canada, writes that he finds it possible to obtain 
measurements of satisfactory accuracy by projecting the image of a window screen 
into the plane of the object. The position of the microscope, at the time of cali¬ 
bration, having been marked upon the work table top by drawing a pencil around 
the base of the microscope. 
