CH. IV] 



MAGNIFICATION AND MICROMETRY 



115 



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 

 especially determined for every optical combination (z. e., objective and 

 ocular), and for every change in the length of the tube of the micro- 

 scope. That is, it is necessary to determine the ocular micrometer val- 

 uation for every condition modifying the real image of the microscope 



(§ 161). 



Any Huygenian ocular (Fig. 30) may, however, be used as a micrometer ocu- 

 lar by placing the ocular micrometer at the level of the ocular diaphragm, where 

 the real image is formed. If there is a slit in the side of the ocular , and the 

 ocular micrometer is mounted in some way it may be introduced through the 

 opening in the side. When no side opening exists the mounting of the eye-lens 

 may be unscrewed and the ocular micrometer, if on a cover-glass can be laid on 

 the upper side of the ocular diaphragm. 



Fig. 104. Fig. 105. 



Figs. 104-105. Ocular Micrometer with movable scale. Fig. 104 is a side view 

 of the ocular while Fig . 105 gives a sectional end view, and shows the ocular mi- 

 crometer hi position . In both the screw which moves the micrometer is shown at 

 the left. (Bausch & Lomb Opt. Co.) 



§ 172. Obtaining the Ocular Micrometer Valuation for an 

 Ocular Micrometer with Fixed Lines (Figs. 104-105). — Use the 

 stage micrometer as object. Light the field well and look into the 

 microscope. The lines of the ocular micrometer should be very sharply 

 defined. If they are not, raise or lower the eye-lens to make them so ; 

 that is, focus as with the simple magnifier. 



When the lines of the ocular micrometer are distinct, focus the 

 microscope (§72, 74, 75) for the stage micrometer. The image of the 

 stage micrometer will appear to be directly under or upon the ocular 

 micrometer. 



Make the lines of the two micrometers parallel by rotating the ocular 

 or changing the position of the stage micrometer or both if necessary, 



