ESTIMATION OF RED CORPUSCLES. 149 



Some i in. 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 hsemocytometer. 



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. 

 If you exclude the spaces which are thus marked with a 

 double line the whole area will be marked out into 

 a series of large squares, each consisting of 4 x 4 = 16 

 smaller squares (fig. 24). It is convenient to count the 

 smaller squares in these groups of 16. At least a hun- 

 dred of the smaller squares, i.e., six of the large groups 

 and four small squares, should be counted. 



In counting one of the smaller squares it is convenient 

 to begin with the corpuscles which are lying in the 

 middle of the square, and then to count those which are 

 lying on the lines. In dealing with these you count 

 those which are lying on the upper and left-hand lines 

 as being within the square, and those that are on the 

 lower and right hand lines as being without it ; if you like 

 you may reverse this, but you must keep to the same 

 method throughout (see fig. 24). 



A few white corpuscles will be met with in every 

 case, while if the blood was taken from a patient with 

 leucocytosis or leucocythaemia there will be many. 

 They may be distinguished from the red corpuscles by 

 their greater refractivity, or, if a stain has been used in 

 the diluting fluid, by their being faintly tinged. It is 



