APPENDIX I 673 
surface of the glass may be etched with squares (0.5 mm.), the 
net micrometer (Fig. 501). 
STANDARDIZATION OF OCULAR MICROMETER 
The value of each division of the ocular micrometer scale must be 
ascertained for each optical combination 
(ocular, objective, and tube length) by the aid 
of a stage micrometer. 
The stage micrometer (Fig. 502) is a slide 
with a scale engraved on it divided to hun- 
dredths of a millimeter (0.01 mm.), in some 
cases, also, to tenths of a millimeter (0.1 mm.), 
every tenth line being made longer than Fic. 501.—Net mi- 
intervening ones, to facilitate counting. crometer, about 173 X 
Miricas actual size. The en- 
ETHOD: 
Ee graved scale is 10 mm. 
1. Insert the ocular micrometer within the square and is divided 
tube of the ocular by placing it on the dia- into small squares with 
: 0.5 mm. sides. 
phragm of the ocular, and adjust the stage 
micrometer by placing it on the stage of the microscope. 
2. Focus the scale of the stage micrometer accurately so that 
the lines of the two micrometers will appear in the same plane. 
Make the lines on the two micrometers parallel each other. 
Fic. 502.—Stage micrometer used to standardize the ocular micrometer. The 
mm. readings etched on this micrometer indicate the width of the larger and smaller 
spaces of the stage micrometer scale seen in the center of the stage micrometer. 
This can often be done by turning the ocular to the right or left, 
while looking into the microscope. 
3. Make two of the lines on the ocular micrometer coincide 
with two on the stage micrometer. Note the number of included 
divisions. 
4. Note the known value for each division of the stage 
micrometer scale which may either be etched on the stage 
