43 MA GNIFICA TION AND DRA WING. 



micrometer on glass, and the lines are sufficiently coarse to be clearly 

 seen by the ocular. The lines should be equidistant and about ^th or 

 2ijth mm. apart and every fifth line should be longer and heavier to facili- 

 tate counting. If the micrometer is ruled in squares (net-micrometer) 

 it will be very convenient for many purposes. 



The ocular micrometer is placed in the ocular, no matter what the 

 form of the ocular (z. e., whether positive or negative), at the level at 

 which the real image is formed by the objective, and the image appears 

 to be immediately upon or under the ocular micrometer and hence the 

 number of spaces on the ocular micrometer required to measure the 

 real image may be read off directly. This is measuring the size of the 

 real image, however, and the actual size of the object can only be de- 

 termined by determining the ratio between the size of the real image 

 and the object. In other words it is necessary to get the valuation of 

 the ocular micrometer in terms of a stage micrometer. 



§ 113. Valuation of the Ocular Micrometer. — This is the value 

 of the divisions of the ocular micrometer for the purposes of microm- 

 etry, and is entirely relative, depending upon 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 specially 

 determined for every optical combination (z. e. , objective and ocular) 

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

 That is, it is necessary to determine the ocnlar micrometer valuation for 

 every condition modifying the real image of the microscope (§ 103). 



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

 ample, employ the 25 mm. ocular and 18 mm. objective. Place 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.* 



Ivight the field well, and look into the microscope. The lines on 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 mi- 



* It is a great convenience to have a micrometer ocular (\ 25) with a spring and 

 screw to enable one to accurately place the ocular micrometer. Any negative ocu- 

 lar may, however, be used as a micrometer ocular by placing the ocular micrometer 

 at the level of the ocular diaphragm, that is where the real image is formed. This 

 is very conveniently arranged for by some opticians by a slit in the side of the ocu- 

 lar, and the ocular micrometer 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. 



