METHOD OF MEASURING MICROORGANISMS 73 



and across which a single-line index is made to travel by the 

 use of the micrometer screw. 



The micrometer screw is fitted with a drum divided into 

 100 parts, one revolution of which moves the index line 

 one division or 5 microns. The drum is divided into fifty 

 parts, so that each mark on the drum scale corresponds to 



5 microns 



or 0.1 micron. 



oU 



The micrometer value of each interval should be cali- 

 brated for each objective with the aid of the object microm- 

 eter. The eye-lens of the micrometer is adjustable to enable 

 the observer to focus the scale accurately. 



The filar ocular micrometer slips into the draw-tube 

 of the microscope like any ordinary ocular and may be 

 fixed in position by the milled-head screw on the side. 



A. Calibration of the Filar Ocular Micrometer. 



Apparatus. Filar ocular micrometer; microscope; ob- 

 ject micrometer. 



Method. 1. Place the object micrometer under object- 

 ive No. 3 and ocular No. 1, drawing out the draw-tube to 

 17 mm. 



2. Bring the lines on the object micrometer into a sharp 

 focus. 



3. Replace ocular No. 1 with the filar ocular microm- 

 eter. 



4. Focus again so that the division of the object microm- 

 eter and the ocular micrometer are equally clear and turn 

 the ocular micrometer so that the lines of both micrometers 

 are parallel to each other. 



5. Determine how many microns one space of the ocular 

 micrometer represents. 



Example. Six divisions of the ocular micrometer-scale cover the 

 same length as three divisions of the object micrometer on which each 

 division is 1/100 millimeter or 10 microns; therefore, six divisions of 

 the ocular micrometer scale equals 30 microns and one division equals 

 1/6 of 30 or 5 microns. 



