METHOD OF PREPARING AND EXAMINING THICK-FILMS 361 



in the thin fihns have learned to do thick films in a day or two. 

 The following directions may be of use to the less experienced. It is 

 assumed that the examiner is already familiar with the appearance 

 of malaria parasites in the thin film. 



Examine thick films of normal blood and get familiar with the 

 appearance of all normal constituents of the blood as they ap- 

 pear in such preparations. IMake thick films of known positives, 

 spreading the blood at one side of the drop until it becomes es- 

 sentially a thin film. Before staining, dry the preparation enough 

 to partially fix the red blood cells at the thinner margin. Compare 

 the appearance of parasites in portions of the film of various 

 thickness. A little practice of this sort will teach the learner more 

 than can be expressed in many paragraphs of description. 



A few general directions may be of particular assistance : 

 Except in the case of crescents, it is unsafe to call anything a 

 parasite unless it shows a red chromatin dot, or mass, associated 

 with blue cytoplasm. The latter, of course, is not always in the 

 form of a ring; it may appear as a round or oval body and is 

 not uncommonly irregular in form. With increasing experience 

 and in clean preparations one can sometimes take into consid- 

 eration pigment alone or pigment associated with cytoplasm only. 



Bacteria sometimes stain like chromatin and may deceive when 

 accidentally associated with a basophilic cloud or a remnant of 

 a leucocyte ; but bacteria are rare in clean preparations and do 

 not show the even distribution throughout the film characteristic 

 of malarial parasites. Red dots or masses, not bacteria, occa- 

 sionally occur evenly distributed in the blood, and in some cases 

 may be the remnants of malarial parasites ; but it is not safe 

 to reckon them as parasites unless they are clearly associated 

 with cytoplasm. These red dots are found in the red cell itself, 

 and in the thick film may lie close to the blue remnant of a baso- 

 philic cell. As in the case of bacteria, one has to be on his 

 guard against regarding such accidental associations as parasites. 

 A good general rule is not to reckon as a parasite anything which 

 can be interpreted as an artifact. The number of doubtful appear- 

 ances will diminish as one gains experience. 



When dirt from the skin is mixed in the thick film, much 

 time is lost in trying to distinguish parasites from bacteria and 

 other debris, and errors in diagnosis are far more likely to occur. 

 If a preparation is very "smudgy," especially if the dirt is dis- 



