136 LABORATORY BACTERIOLOGY 



II 



THE OCULAR MICROMETER AND MICROMETRY 1 



" 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 j'j or 

 ^ 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 (i.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 F'^ld of large filar mi- 

 object. In other words, it is necessary to crometershowing cross 



^ . . hajrs 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 {i.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, 

 pubhshed here by his consent. The references to sections are to the seventh 

 edition of The Microscope. 



^ " I JP I " ^ 



