70 GENERAL MICROBIOLOGY 



EXERCISE 19. METHOD OF MEASURING MICRO- 

 ORGANISMS 



1. Using the Leitz Ocular " Step " Micrometer. In 

 this ocular micrometer the intervals are arranged in groups 

 of ten, each group being indicated by black steps rising 

 from the first to the tenth interval. 



This arrangement possesses the great advantage that the 

 divisions can always be seen distinctly whether the objects 

 be light or comparatively dark. 



The intervals of the scale, instead of being 0.1 mm. or 

 0.5 mm. wide, as in ordinary ocular micrometers, have a 

 definite value of 0.06 mm. This gives for each objective 

 and for a given tube length, convenient and in many cases 

 integral micrometer values, which renders a greater facil- 

 ity in the use of this instrument. The actual tube length 

 differs in most cases but little from the standard length. 

 The tube length and the micrometer value of each micro- 

 scope, however, should be separately calibrated. 



It is of importance to be able to determine the size of 

 microorganisms: (1) because it is of general interest to 

 know the size of the microorganisms with which we are 

 dealing; (2) because the difference in size is an important 

 factor in' identifying and describing the organism; (3) 

 because the size is necessary for purposes of comparison 

 with other microorganisms. 



Apparatus. Microscope; Leitz ocular " step " microm- 

 eter; object micrometer; specimen to be measured. 



Method. 1. With the aid of the Leitz ocular " step " 

 micrometer the size of stained or unstained microorganisms 

 on either a light or a dark field may be measured directly 

 in microns. 



A micron is 0.001 mm., and is expressed by the Greek 

 letter p.. 



2. One hundred divisions of the step micrometer cover 

 100, 15 and 10 divisions of the object micrometer 



