174 



Filter, and the solution will be ready for use. The 

 white blood cells are stained violet, and may thus be 

 counted with the red. 



The diluting fluid contained in the capillary tube is 

 then blown out, and a small drop of the diluted blood is 

 placed on the centre of the small glass disc. This small 

 disc is surrounded by a ring of glass, cemented to the slide. 

 The glass ring is 0.1 m. m. thicker than the glass disc. 

 When this small moist chamber is covered with a thick 

 cover glass, we have a layer of blood 0.1 m. m. deep be- 

 tween the disc and the cover glass. On the upper surface 

 of the small glass disc (on which the drop of diluted blood 

 was placed) there are marked off 400 small squares. The 

 sides of the small squares are ^ of i m. m. long. It will 

 be seen that the layer of blood over each of the squares 

 would have a cubical contents of 



4oW of a c. m. m. ( ' T x ^ x T V = ^). 



The haemacytometer slide is now placed on the stage of 

 the microscope, where it should remain undisturbed for 

 several minutes before counting. The red blood cells in 

 25 to 50 squares are then counted. To ascertain the num- 

 ber of red cells in a cubic millimeter the following formula 

 may be useful : 



