96 METHODS OF OBSERVING BACTERIA 



equal spaces. The steps in the use of the two micrometers for the 

 purpose of measuring bacteria, cells, and other very small micro- 

 scopic objects are as follows: 



1. Unscrew the upper lens of the eyepiece, which exposes the 

 diaphragm of the latter. Place the circular glass disk micrometer, 

 generally held in a small metallic frame, with its flat side downward 

 upon the diaphragm, replace the front lens of the eyepiece and return 

 to its proper position on the draw tube of the microscope. Regulate 

 the upper lens of the eyepiece so that the lines of the eyepiece 

 micrometer show clearly. 



2. Place the stage micrometer on the stage and get its scale in 

 focus with the oil-immersion objective. This must be done according 

 to the rules for examining unstained objects, hence the .iris diaphragm 

 must be closed as much as possible. 



3. The scale of the stage micrometer runs from above downward 

 in the field of vision. Now manipulate the eyepiece micrometer, 

 by rotating the eyepiece, so that the ruled scale of the latter also 

 runs from above downward. Next, move the stage micrometer (this 

 can be more easily done if the microscope has an attachable mech- 

 anical stage) so that the first line of its scale and the first line of the 

 eyepiece micrometer fall together (overlap each other). 



4. Manipulate the draw-tube so that a number of subdivisions 

 of the eyepiece micrometer are just equal to one partition that is, 

 the space between two lines of the stage micrometer. 



5. Suppose that eight of the spaces of the eyepiece micrometer 

 are equal to one space of the stage micrometer; then each space of 

 the eyepiece micrometer with the magnification used is equal to 

 1.25 micra. As already stated, the stage micrometer is so ruled that 

 1 mm. is divided into 100 equal parts; therefore, each space is equal 

 to 10 micra, and 8 eyepiece spaces =10 micra; hence, 1 space= 1.25 

 micra. 



6. Raise the tube of the microscope on the coarse adjustment; 

 remove the stage micrometer and replace it by the stained cover- 

 glass preparation of bacteria which are to be measured. Open the 

 iris diaphragm and bring the bacteria into focus. 



7. Now manipulate the slide on the stage and the eyepiece by 

 rotating it so that a typical bacterium, say a bacillus, just stands at 

 right angles to the lines of the ruling of the eyepiece micrometer. 

 See that the end of the bacterium apparently just touches one of the 

 lines and note over how many divisions the bacillus extends. 



8. Suppose that the bacillus just fills three divisions of the scale; 

 then it is three times 1.25 micron or 3.75 micra long. 



9. In making these measurements the following precautions are 

 to be used. The slide on which the bacterial preparation is made 

 should be the same thickness as the stage micrometer slide. After 

 the draw-tube has been adjusted in order to bring a whole number 

 of divisions of the eyepiece into one division of the scale of the stage 



