40 PRACTICAL HISTOLOGY. 



it should invariably be tested by a practised observer. Clear 

 daylight from a white cloud if possible should always be 

 employed, and its regulation by the diaphragm carefully 

 attended to. The cover-glass should be extra thin and 

 perfectly clean. The salivary corpuscle is a good test- 

 object for -inch lens. If the margin of the nucleus, and 

 of the corpuscle, and the Brownian motion of the granules 

 ( 86, b), cannot be sharply seen, the definition is imper- 

 fect. If the corpuscle, when exactly in focus, shows any 

 colour, the microscope is not achromatic. A drop of blood 

 should also be used. It should form a very thin layer 

 under the cover-glass. If, when the tube of the microscope 

 is fully elongated, the corpuscles in the centre of the field 

 are sharp, while those near the periphery are misty, the field 

 is not flat. The adjustments must work smoothly. The 

 stand of the microscope must be perfectly steady, and the 

 objective must on no account move from side to side when 

 the fine adjustment is used. A microscope should be rejected 

 without hesitation for any one of these faults. The fine 

 markings upon the scales of Lepisma saccharina, Podura 

 plumbea, various diatoms, and Nobert's test-object con- 

 sisting of parallel lines of various degrees of approximation 

 ruled upon glass, are all recommended as test-objects. 

 Doubtless, they are good tests of defining power, but they 

 are well-nigh useless as tests of achromatism, because of 

 the dispersion which they themselves occasion. 



THE METHOD OF WORKING WITH* THE MICROSCOPE. 



55. Preparation of the Microscope. a. Select the 

 objective and ocular required, and place them on the 

 microscope. In the present case the -^-inch lens (No. 7 of 

 Hartnack) and an eye-piece of medium power (No. 3 of 

 Hartnack) will be needed. See that their outer surfaces 

 are clean, if not, rub them gently with soft wash-leather. 

 If they are difficult to clean use dilute ammonia. 



b. If the objective be the ^-inch (No. 7 of Hartnack), 

 place it half-an-inch above the level of the stage ; if it be 

 the i -inch (No. 3 of Hartnack), place it two inches above the 

 stage. In these positions they are of course outside the focus. 



