144 BACTERIOLOGICAL DIAGNOSIS. 



on the instrument (fig. 23, a). This is ruled in squares, 

 and the squares are of known size. The amount of 

 blood lying upon each square is thus known, and the 

 number of corpuscles which lie upon it being counted 

 under the microscope, all- the data for the calculation 

 are obtained. 



In blood examinations it is absolutely necessary that 

 all points in the technique should receive the most 

 careful attention, or the result will be worse than 

 useless. For this reason we shall describe each step in 

 the process at some length, and advise the practitioner 

 to make several estimations before placing any reliance 

 whatever on his results. 



Requisites. i. The haemocytometer. 



2. A needle suitable for obtaining a small quantity of 

 blood- A Hagedorn's needle is the very best that can 

 be used, and an ordinary triangular surgical needle will 

 answer very well. 



3. Diluting fluid. There are a good many formulae 

 for this, and some are rather complicated. Isotonic 

 saline solution (common salt -5 per cent.) will answer 

 perfectly ; it is advisable to add to it a small quantity 

 of some stain, methyl violet being the best, although 

 gentian violet will do very well. This colours the leuco- 

 cytes, so that they are readily distinguished from the 

 red corpuscles.* 



4. A microscope having an -| inch lens which will 

 focus through the thick cover-glass supplied with the 

 haemocytometer. If the examination is not to be made 

 by the bedside a strong india-rubber band a little 

 shorter than the pipette should be carried. 



* The following formula is better: Distilled water, 160 c.c.; 

 glycerine, 30 c.c. ; sodium sulphate, 8 grams ; sodium chloride, i 

 gram ; methyl violet, a trace. 



