128 BACTERIOLOGY. 



mmute^t details which makes an instrument truly valuable. 

 The properties which a good objective should possess are 

 as follows: 



1. — A proper magnifying power. — In the case of a single 

 lens the magnifying power may be said to depend upon its 

 focal distance. The shorter the focal length the higher the 

 magnification. Therefore, a lens with a one-fourth inch 

 focus will magnify more than one having a focal distance 

 of one inch. In the case of the objective, we have to deal 

 not with one lens, but with a combination of lenses, and it 

 is customary then to speak of the "equivalent focal dist- 

 ance. " This represents what would be the focal distance 

 of a simple lens having the same magnification as the ob- 

 jective. When, therefore, a lens is designated as a one- 

 fourth inch objective, it does not mean that the dist- 

 ance from the object to the front lens is one-fourth of an 

 inch. On the contrary, this "working distance," as it is 

 called, would be much less than a fourth of an inch. The 

 one-fourth inch objective, however, does possess the same 

 magnification as a single lens having a focal distance of 

 one-fourth, of an inch. 



A table showing the magnifying power of the several 

 objectives and eye-pieces is usually supplied by the instru- 

 ment m"aker. The magnifying power, of a given objective 

 and eye-piece, can be obtained by the following simple 

 method. . A stage micrometer, or object of known length, is 

 necessary. The micrometer is a glass slide on which an 

 accurate scale has been cut. The scale may consist of 

 one _ mm. divided into 100 parts. The micrometer is ad- 

 justed. on the stage of the microscope. The draw- tube of 

 the latter should be drawn out, vertically, so that the 

 upper surface of the eye-piece is 10 inches from the table. 

 On looking into the microscope, both eyes kept open, the 

 image of the micrometer will be seen to be projected on the 

 table. The limits of the projected scale can be marked on 

 the table, and the distance then measured. If, for exam- 



