MICROMETRY — MICROMETRIC MICROSCOPES 
183 
mination of the ocular micrometer value may be made more 
exact by eliminating fractions as shown in Fig. 116/ where it is 
evident that a whole 
number of ocular scale 
divisions are included 
in a whole number of 
divisions of the stage 
micrometer. This is 
accomplished by alter¬ 
ing the ratio between 
the images of the two 
scales through a change 
in the position of the 
draw-tube. Start with 
the draw-tube extended 
about half its total pos¬ 
sible movement. Bring 
. ... Fig. 116. Micrometer Scales Properly Adjusted, 
the zero point of the 
ocular scale in contact with a line on the stage scale; focus 
sharply. The relations of the images of the two scales will 
now probably be essentially as indicated in Fig. 115. Note the 
magnitude of the distance of the extreme line on the ocular 
scale (50 on the B. & L. ocular) and the nearest line on the 
stage micrometer. With this magnitude clearly in mind, push 
in the draw-tube about 2 millimeters, focus and note whether 
the ocular line (50) is now nearer or farther away than before. 
If nearer push in the draw-tube a little more, focus and again 
note the change. Keep this method up until both the zero 
line and the farthest line on the ocular scale are each in con¬ 
tact with lines on the stage scale. It is not essential that coin¬ 
cidence must obtain in each ten divisions as indicated in Fig. 
116. If on the other hand, the first change in draw-tube length 
^ Figs. 115 and 116 were drawn by means of a camera lucida and therefore show 
exactly the conditions met with. Each division on the stage micrometer (the 
lines crossing the entire field) equals o.i mm. With a pair of dividers compare 
the magnitude of a space in Fig. 115 with one in Fig. 116. It will be found that 
lengthening the draw-tube has changed the ratio between images of stage and 
ocular scales. 
