52 



EXPERIMENTAL DAIRY BACTERIOLOGY 



ocular. The glass bears a scale made up of lines at an equal 

 but not necessarily known distance apart. The scale of the 

 micrometer should lie in the plane of the real image formed 

 by the objective. The image of the object thus lies on the 

 scale, and may be measured by it if the value of each di- 

 vision of the scale be determined in actual units of measure- 

 ment. For this purpose a stage micrometer is necessary, a 

 slide bearing a cover glass on which is ruled a scale in 0.1 

 and 0.01 mm. The ratio of ocular divisions to the image of 

 the stage divisions is determined by focusing with the ocular 

 micrometer in place on the scale of the stage micrometer. 

 The lines of the two micrometers are made parallel by rotat- 

 ing the ocular. Make the image of any two lines on the stage 

 micrometer coincide with two lines on the ocular microm- 

 eter, by drawing out the tube of the microscope if neces- 

 sary. From the data thus obtained it is easy to calculate 

 the value of each space on the ocular micrometer in 0.01 of 

 a millimeter. The ocular used, the objective, the number of 

 the ocular micrometer, and the tube length of the microscope 

 must be recorded and also the value of a single space on the 

 ocular micrometer for the given system of lenses in microns 

 (0.001 mm.) expressed by the Greek letter /*. 



Exercise. Determine the value of the ocular micrometer and fill 

 out the blanks in the following table : 



No. of microscope- 



Make. 



