MICROMETRY — MICROMETRIC MICROSCOPES 
189 
to a cross-bar sliding upon a strip of wood 25 cm. long graduated 
in centimeters; this strip is attached to two small blocks, each 
the thickness of the base of the microscope stand. One block 
is notched at the end so as 
to permit its being always 
placed exactly in the same 
positive (Ives’ method). 
The best results are ob¬ 
tained when a strong source 
of artificial light is employed 
to illuminate the screen and 
a piece of ground glass is 
placed between the radiant 
and the scale-screen. 
The values of the scale Fig. 122. Full Size Scale for Micrometry 
are determined for three or Projection of Image by the Substage 
. . . Condenser. 
more positions of the gradu¬ 
ated strip and the results plotted upon coordinate paper. 
This is accomplished as follows; Place a stage micrometer upon 
the stage of the microscope, center and focus sharply using say 
a 16 mm. objective and 7.5 X eyepiece. Raise the substage con¬ 
denser until the upper lens almost touches the object slide; open 
the iris diaphragm. Tip the plane mirror to one side and at the 
proper angle to throw an image of the scale into the condenser. 
Lower the condenser while looking into the microscope until 
the scale becomes clear and sharp. Turn the stage micrometer 
so that its graduations become parallel with those of the real 
image of the scale-screen. Move the stage micrometer until any 
line of the stage micrometer coincides with a line on the scale 
image. Count the number of division of the scale included in 
a division of the stage micrometer. Calculate the value for 
one division of the scale. Record the distance of the scale from 
the mirror as shown on the graduated strip and compute the 
value in microns as obtained for this position. Move the scale 
carrier to a new position and determine the value of a scale divi¬ 
sion as described above. In like manner find the true value for 
a third position. Plot the results upon a fairly large sheet of 
