20 



A MANUAL OF BACTERIOLOGY 



The above will serve as approximately correct 

 tables for ordinary work, but if the exact magnifying 

 power of any objective is required it must be 

 specially tested. 



The proper illumination of microscopic objects is 

 of the highest importance, and that suitable for one 

 class may be altogether unfit for another. Daylight 

 is the best light to use for bacteriological work ; 

 but if one is working at night or in the winter, a 

 paraffin lamp is required. It is essential that the 

 flame should be steady and of moderate size. 

 Parallel rays may be advantageously thrown on the 

 mirror (Fig. SD) of the microscope by means of a 

 bull's-eye condenser, placed so that the flame is 

 nearly in the focus. For comparatively low powers, 

 a fla.t or concave mirror may be used to reflect the 

 light, but for higher powers it is essential that the 

 light should be concentrated by means of an Abbe's 



