PRACTICAL HISTOLOGY. 



By the above method the magnifying power can be 

 ascertained with perfect accuracy; there is, however, an 

 easier way, which may be had recourse to by those who 

 possess a simple prism or a parallel plate reflector ( 58, 

 b and c). (Chevalier's camera is not here admissible, as it 

 involves an increase of magnifying power by lengthening 

 the tube.) If a scale be held ten inches in front of the 

 prism, or the same distance below the parallel plate, the 

 extent to which the micrometer divisions are magnified 

 may be readily ascertained. 



Having measured the power with the tube elongated, 

 shorten it and estimate the power again. It will be found 

 that the shorter the tube, the lower is the power. In this 

 manner the powers of all the possible combinations of 

 objectives and oculars belonging to a microscope should 

 be ascertained and tabulated as follows.* 



) Tube 

 ) elongated. 



The maker of the microscope generally supplies a table 

 such as the above ; but it is essential that the student should 

 know how it is constructed, in order that he may test its 

 accuracy, and measure the power of any lens that he may 

 add to his microscope. 



60. Estimation of the Size of a Microscopic Ob- 

 ject. This is done most easily and most accurately with 

 an eye-piece micrometer (Fig. 40). This consists of a 

 piece of glass with a fine scale engraved on it. It is most 

 convenient to have a special eye-piece containing a micro- 

 meter, but it is also possible to get a micrometer adapted 

 to the ordinary eye-piece. The ocular micrometer may be 



* This table applies to a Hartnack's microscope, but it serves to 

 indicate how the powers of any form of microscope may be tabulated. 



