98 PRACTICAL HISTOLOGY. [l. 



The rule with regard to the use of the diaphragm must never be 

 neglected, viz., to use a small aperture with a high power, and a 

 large aperture with a low power. 



2. Clean a Slide and Cover-Glass. 



(a.) The Slide. Seize the slide by its edges with the thumb and 

 forefinger of the left hand, dip one half of it into water, withdraw 

 it, and with a clean old handkerchief rub both wetted surfaces at 

 once until they are clean and dry. Reverse the slide, still holding- 

 it by its edges, and dip the other end in water, and clean its surfaces 

 as before. Lay the slide upon some clean, suitable background, 

 white or black paper, or on the photophore. 



(/;.) The Cover-Glass. Sometimes the covers have a thin film on 

 them ; this may be got rid of by placing them in strong sulphuric 

 acid, and subsequently removing every trace of acid by water. Dip 

 the cover-glass in water, take it between two folds of a handkerchief 

 held between the thumb and forefinger of the right hand, and rub 

 both surfaces at once. After it is cleaned, do not lay it flat, but 

 tilt it up against some convenient object. 



The first lesson is devoted to the examination of a few simple 

 objects some of which are occasionally found as foreign bodies in 

 microscopical preparations with a view to familiarise the student 

 with the use of the microscope. 



3. Milk. By means of a glass rod, place on the centre of the 

 slide a small drop of milk diluted with three or four volumes of 

 water. To find the centre of the slide, use the mounting block 



(P- 89). 



Apply a Cover-Glass. Seize the cover-glass by the edge by means 

 of a pair of forceps with broad points. The pattern shown in fig. 

 .6 is convenient. The edge of the cover-glass opposite to the forceps 

 is allowed to touch the slide close to the drop of fluid, the edge 

 opposite being gradually and evenly lowered by depressing the 

 forceps until the fluid touches the under surface of the cover-glass. 

 By lowering the cover-glass thus gently and obliquely the entrance 

 of air-bubbles is avoided. Place the object on the stage right under 

 the lens. 



Focut tJte Object (H). The objective is still half an inch above 

 the stage. While looking into the eye-piece of the microscope, 

 seize the tube of the latter between the thumb and adjoining fingers 

 of the right hand, and with a screwing or twisting movement of 

 the tube from left to right, gradually depress the tube until the 

 outlines of the object are indistinctly seen. This is the coarse 

 adjustment. The focussing process is facilitated by keeping the 

 slide and object moving slightly. This can readily be done by 

 moving the slide with the thumb and forefinger of the left hand, 

 the ulnar margin of the palm conveniently resting on the table. 



