ESTIMATION OF THE RED CORPUSCLES 



241 



board (Fig'. 46). Get these squares into the centre of the 

 field (see Fig. 48). 



Do not forget you are dealing with an unstained object; 

 use a flat mirror and a small diaphragm. The examination 

 is often easier if artificial light is used. 



Now turn on the high power (J-inch or i-inch), and screw 

 it downwards until it almost touches the cover-glass; look 

 down the microscope and focus gently upwards, using the 

 fine adjustment, and keeping a careful look-out for the 

 rulings. 



If the rulings of the slide are indistinct, they may be dark- 

 ened by rubbing" them with a very soft lead-pencil. 



FIG. 47. COUNTING CHAMBER WITH COVER GLASS CLIPPED INTO POSITION. 



5. Counting the Corpuscles. Move the slide about until 

 you have come to one corner (preferably the left upper 

 corner) of the ruled area. You will see that each fifth space 

 is marked off by a line running down its centre; this is to 

 guide the eye and facilitate counting. The whole square 

 consists of four hundred small squares, twenty along each 

 side. You have to count at least a hundred of these small 

 squares. The simplest way to do this is to count five "bars" 

 of twenty each, each bar extending right across the ruled 

 "chess-board." The bars selected should be as far as pos- 

 sible apart from one- another, so as to get a good averag'e. 



16 



