ch. /r.] 



MAGNIFICATION AND MICROMETRY. 



103 



and for every change in the length of the tube of the microscope. That 

 is, it is necessary to determine the ocular micrometer valuation for every 

 condition modifying the real image of the microscope (§ 152 ). 



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

 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 un- 

 screwed and the ocular micrometer, if Jon a cover-glass, can be laid on the upper 

 side of the ocular diaphragm. 



Figs. 98, 99. Filar Ocular Micrometer with Field | Bausch & Lomd, Optical 

 Co. ). For other ocular micrometers sec pp. 25, 26. 



z 163. Obtaining- the Valuation of the Filar Micrometer. This micrometer 

 (Fig. 98-99) consists of a Ramsden's ocular and cross lines. As seen in Fig. 98 

 there are three lines. The horizontal and one vertical line are fixed. One vertical 

 line may be moved by the screw back and forth across the field. 



For obtaining the valuation of this ocular micrometer an accurate stage mi- 

 crometer must be used. Carefully focus the ,,', ,,thmm. spaces. The lines of the 

 ocular micrometer should also be sharp. If thev are not focus them by moving 

 the top; of the ocular up or down (z 164). Make the vertical lines of the filar mi- 

 crometer parallel with the lines of the stage micrometer. Take the precautions 

 regarding the width of the stage micrometer lines given in z 167 (see also Fig. 

 101 I. Note the position of the graduated wheel and of the teeth of the recording 

 comb, and then rotate the wheel until the movable line traverses one space on the 

 stage micrometer. Each tooth of the recording comb indicates a total revolution 

 of the wheel, and by noting the number of teeth required and the graduations on 

 the wheel, the revolutions and parts of revolutions required to measure the iJ-^th 

 mm. of the stage micrometer can be easily noted. Measure in like manner 4 or 5 

 spaces and get the average. Suppose this average is i' + th revolutions or 125 

 graduations on the wheel, to measure the njjtb mm. or tOjU (see # 157), then one 

 of the graduations on the wheel would measure [O/i divided by 125 ---.08//.. In 



