APPARATUS FOR HOLDING SPECIMENS 



59 



the red or white, or with a solution of acetic acid when a count 

 of white cells only is being made. In counting red corpuscles 

 a dilution of 1-200 is generally used, but in certain cases 1-100 

 may be employed. The blood is drawn into the pipette up to the 

 mark 0*5 in the case of 1-200 dilutions, and up to the mark 1 for 

 1-100 dilutions. The pipette is then immediately placed in the 



Fia. 60. — No. 3325a, Pipettes for Red and White Corpusdes. 



diluting fluid, which is drawn up to the mark 101 above the bulb. 

 Both ends of the pipette are then closed with the fingers, and the 

 pipette shaken to ensure an even mixing, the glass bead in the 

 bulb facilitating this. For white corpuscles, a dilution of 1-10 is 

 employed and the other pipette is used. For filling the counting 

 chamber, a drop of the mixture is blown out of the pipette, after 

 allowing several drops to go waste, into the centre of the counting 

 chamber. The cover glass is then placed over the cell. The drop 

 of blood must not be allowed to overflow the platform into the 

 groove which surrounds it, and the cover glass must be in perfect 

 contact with the object slide, and aU must be scrupulously clean. 



B 



I 



-Uj- 



C 



_i_ 



1 



Fig. 61. 



Fig. 62. — No. 3325a, Thoma-Hawksley Counting Chamber. 



The counting chamber consists of a plate of glass with an 

 aimular groove ground upon it. The circular portion inside the 

 groove is ground and polished to a distance of '1 mm. below the 

 level of the plate of glass. 



In the Thoma hsemacytometer this portion is ruled with a 

 diamond into squares l/400th of a square mm. each in area. It 



