STAINING OF PARASITES 15 



Preparation of Blood Films. The essential in the pre- 

 paration of blood films for examination of the parasites 

 is that the film should be so thin that the red corpuscles 

 lie flat over a considerable part of the film. With fresh 

 blood this result can be attained if the slides and cover- 

 glasses are free from grease and grit so that the blood 

 can run rapidly ; and if the drop of blood is so small 

 that it does not fill the whole space between the slide and 

 cover-glass, the edge of the film will always be too 

 thick and the centre will contain too few corpuscles. 



FIG. 4. Indicating the arrangement of the red cells in the different parts 

 of a fresh fluid film. (a) Edge where corpuscles overlap each other, so 

 that parasites in them are not readily seen, (b) Centre so thin that the red 

 cells are scanty, (c) The best part of the film for examination. Red cells 

 numerous and all flat, so that parasites are easily found and seen. 



The space between should look opalescent and in it the 

 corpuscles lie side by side and flat (fig. 4, c). Dried films 

 are best made with two slides. The drop of blood should 

 be taken up on the extreme edge of the lower surface of 

 one slide, and then this slide brought into contact with 

 the upper surface of a second slide at an angle of about 

 45. The blood will run along the edge of contact, and if 

 the upper slide is pushed so as to glide over the surface 

 of the lower slide, a film of blood suitable for examina- 

 tion will be left behind (fig. 5). 



