CH. IV.] 



MAGNIFICATION AND MICROMETRY. 



103 



and for every change in the length of the tubs 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 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. This is very conveniently arranged for by 

 some opticians by a slit in the side of the ocular, and the ocular mi- 

 crometer is mounted in some way and simply introduced through the 

 opening in the side. When no side opening exists the mounting of the 

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

 can be laid on the upper side of the ocular diaphragm. 



It is a great convenience to have a micrometer ocular with a spring 

 and screw to enable one to accurately place the ocular micrometer. 

 The screw or filar ocular micrometers are excellent and for some pur- 

 poses almost indispensable (Fig. 100). They are expensive, however, 

 and for ordinary measurement unnecessary. If one is to do much meas- 

 ureing it is almost a necessity to have an ocular micrometer with screw 

 for moving the scale, (Figs. 98-99). 



Fig. 9.8. 



Fig. 99. 



Figs 98-99 Ocular Micrometer with movable scale. Fig. pS is a side view of 

 the ocular zvliile Fig. 99 gives a sectional e?id view, and shows the ocular microme- 

 ter in position. In both the screw which moves the micrometer is shown at the 

 left. (From Biusch & lomb. Opt. Co. ) 



§ 163. Obtaining the Ocular Micrometer Valuation. — As an ex- 

 ample, employ the 25 mm. ( 1 in.) ocular and 16 mm. (J/s in. ) objec- 

 tive. Piace the stage micrometer under the microscope for an object, 

 and put the ocular micrometer in position, either through a slit in the 

 ocular, or remove the eye-lens and place it upon the ocular diaphragm. 



Light the field well, and look into the microscope. The lines on the 

 ocular micrometer should be very sharply defined. If they are not, 



