ESTIMATION OF THE RED CORPUSCLES 187 



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. 



Some i-inch lenses focus too near the object to be of any use. 

 If this is the case, you must either get an objective specially for 

 the purpose, or a cover-glass which is hollowed out in the centre. 

 These can be bought from the same place as the haemocytometer. 



FiQ. 46. The "Bars" which are to be Counted are Shaded. 



If the rulings of the slide are indistinct, they may be darkened 

 by rubbing them with a very soft lead-pencil. 



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 



