144 LABORATORY BACTERIOLOGY 
II 
THE OCULAR MICROMETER AND MICROMETRY? 
“Ocular Micrometer, Eye-piece Micrometer. — This, as the name 
implies, is a micrometer to be used with the ocular. It is a microm- 
eter on glass, and the lines are sufficiently coarse to be clearly seen 
by the ocular. The lines should be equidistant and about y, or 
zy mm. apart, and every fifth line should be longer and heavier to 
facilitate counting. If the micrometer is ruled in squares (net 
micrometer) it will be very convenient for many purposes. 
The ocular micrometer is placed in the ocular, no matter what the 
form of the ocular (z.e. whether positive or negative), at the level 
at which the real image is formed by the objective, and the image 
appears to be immediately upon or under the ocular micrometer, and 
hence the number of spaces on the ocular 
micrometer required to measure the real 
image may be read off directly. This is 
measuring the size of the real image, how- 
ever, and the actual size of the object can 
only be determined by determining the ratio 
between the size of the real image and the Field of large filar mi- 
object. In other words, it is necessary to bee gpl gl 
i" i airs and recording 
get the valuation of the ocular micrometer comb, 
in terms of a stage micrometer. 
Valuation of the Ocular Micrometer.— This is the value of the 
divisions of the ocular micrometer for the purpose of micrometry, 
and is entirely relative, depending upon the magnification of the real 
image formed by the objective; consequently it changes with every 
change in the magnification of the real image, and must be spe- 
cially determined for every optical combination (¢.e. objective and 
ocular) and for every change in the length of the tube of the mi- 
croscope, that is, it is necessary to determine the ocular microm- 
eter valuation for every condition modifying the real image of the 
microscope (152). 
1 These paragraphs are from Professor S. H. Gage’s work on the microscope, 
published here by his consent. The references to sections are to the seventh 
edition of Zhe Microscope. 
