THE BACTERIOLOGICAL LABORATORY 19 



lively. The medium objectives are issued in two 

 different forms, with a greater or less aperture 

 according to the purpose for which they are required. 

 Those with a large aperture (distinguished by double 

 lettering), possess with equally perfect definition a 

 considerably higher resolving power, and permit 

 of greater magnification being obtained by the use 

 of the stronger eye-pieces. Nevertheless, the work- 

 ing distance in BB, CO, DD, although relatively 

 large, is perceptibly less than in the corresponding 

 series of smaller aperture, and the former are more 

 sensitive to differences in thickness of the cover-glass 

 and object than the latter. Therefore, B, C, and D 

 are recommended as the more suitable for working 

 glasses in histological and anatomical research, 

 particularly when the next stronger dry lens is 

 available for higher magnification. 



The magnifications of the English objectives and 

 eye-pieces with a ten-inch tube (i.e. the English 

 microscope with a long tube), are given in the 

 following table : 



