The microscope; an introduction to microscopic methods and to histology . micrometry, and is entirely relative, depending upon the magnificationof the real image formed by the objective, consequently it changeswith every change in the magnification of the real image, and must beespecially determined for every optical combination (z. e., objective andocular), 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
The microscope; an introduction to microscopic methods and to histology . micrometry, and is entirely relative, depending upon the magnificationof the real image formed by the objective, consequently it changeswith every change in the magnification of the real image, and must beespecially determined for every optical combination (z. e., objective andocular), 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, wherethe real image is formed. If there is a slit in the side of the ocular , and theocular micrometer is mounted in some way it may be introduced through theopening in the side. When no side opening exists the mounting of the eye-lensmay be unscrewed and the ocular micrometer, if on a cover-glass can be laid onthe upper side of the ocular Fig. 104. Fig. 105. Figs. 104-105. Ocular Micrometer with movable scale. Fig. 104 is a side viewof 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 atthe left. (Bausch & Lomb Opt. Co.) § 172. Obtaining the Ocular Micrometer Valuation for anOcular Micrometer with Fixed Lines (Figs. 104-105).—Use thestage micrometer as object. Light the field well and look into themicroscope. The lines of the ocular micrometer should be very sharplydefined. 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 themicroscope (§72, 74, 75) for the stage micrometer. The image of thestage micrometer will appear to be directly under or upon the ocularmicrometer. Make the lines of the two micrometers parallel by rotatin
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