144 MEASURING WITH THE MICROSCOPE [Ch. V 



be determined by determining the ratio between the size of the real 

 image and the object. In other words, it is necessary to get the valua- 

 tion of the ocular micrometer in terms of a stage micrometer. 



§ 239. Valuation of the ocular micrometer. — This is the value of 

 the divisions of the ocular micrometer for the purposes of micrometry, 

 and is entirely relative, depending on 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 change modifying the real image of the micro- 

 scope (§ 235). 



It will be seen when the ocular micrometer valuation is found for 

 different objectives, that the greater the magnification of the objective 

 the less will be the ocular micrometer valuation; and conversely, 

 the less the magnification of the objective the greater will be the ocular 

 micrometer valuation. 



§ 240. Obtaining the ocular micrometer valuation for an ocular 

 micrometer with fixed lines. — If the ocular micrometer is on a cover- 

 glass, place it on the diaphragm of the 5X or iox ocular after removing 

 the eye-lens. Screw the eye-lens back in place, and put the ocular 

 in the tube of the microscope. Put a 16 mm. objective in place. 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 (§ 234) for the stage micrometer. The image of the 

 stage micrometer appears 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, and then make any two lines of the stage micrometer coin- 

 cide with any two on the ocular micrometer (fig. 90). To do this it 

 may be necessary to pull out the draw-tube a greater or less distance. 

 See how many spaces are included in each of the micrometers (see 

 fig. 90, 98). 



Divide the value of the included space or spaces on the stage microm- 



