APPENDIX 



137 



Any Huygenian ocular 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 on a cover-glass, can be laid 

 on the upper side of the ocular diaphragm. 



Obtaining the Valuation of the Filar Micrometer. — This microm- 

 eter (Figs. 98-99) consists of a Ramsden's ocular and cross lines. 



Filar Micrometer. 



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 micrometer must be used. Carefully focus the yJo mm. spaces. 

 The lines of the ocular micrometer should also be sharp. If they 

 are not, focus them by moving the top of the ocular up or down 

 (164). Make the vertical lines of the filar micrometer parallel 

 with the lines of the stage micrometer. Take the precautions 

 regarding the width of the stage micrometer lines given in 167. 



