I2 MICROSCOPIC EXAMINATION OF BACTERIA. 
ratus, the following fairly accurate methods are 
recommended : 
I. First Method. 
(a) Place a micrometer eyepiece in the micro- 
scope. 
(6) Examine the bacteria to be measured, and 
record their lengths in divisions of the eyepiece 
micrometer. 
(c) Remove the slide of bacteria. 
(d) Place a stage micrometer on the stage of 
the microscope, and determine the relation of the 
divisions of the eyepiece micrometer to the. divis- 
ions of the stage micrometer. 
(e) The length of the divisions of the stage mi- 
crometer is a fixed quantity (usually 737 mm.) ; 
therefore we have the equations : 
Length of bacteria = x divisions of eyepiece 
micrometer ; 
x divisions of eyepiece micrometer = y di- 
visions of stage micrometer ; 
I division of stage micrometer = 745 mm. 
From these equations we can readily determine the 
length of the bacteria in millimeters. The unit of 
length for microscopic measurements is the thou- 
sandth part of a millimeter ; it is called a micron or 
micromillimeter, and is designated by the Greek 
letter u. Therefore in the above equations we can 
substitute for zi; mm. Io #, and write the result in 
so many #. 
2. Second Method. 
(a) Adjust the ‘‘camera lucida’’ to the micro- 
scope. 
