36o RESEARCH IN PROTOZOOLOGY 



thin films. As a routine, thin films should he fixed with methyl 

 alcohol before staining. But thin films long dried will afiford 

 good specimens for diagnosis without fixation. 



Quick staining of thick films for early diagnosis. Spread the 

 blood, or a part of it, a little more thinly than for ordinary thick 

 films. Dry. Lay the slide flat, film side up, and pour on a gener- 

 ous quantity (3 or 4 cc.) of freshly diluted Giemsa stain. Stain 

 fifteen or twenty minutes. Wash with water cautiously so as not 

 to loosen the film. Dry and examine. Films brought in one or two 

 at a time for diagnosis are often best stained by this method, 

 especially a kind intended for a thin film but having a surplus heap 

 of blood at one end or the other of the thinner part. 



Preservation of films after examination. If it is desired to pre- 

 serve a preparation, warm the slide, wash off the immersion oil 

 with xylol, and quickly wash off the xylol with absolute ethyl 

 alcohol. Blot or dry quickly. Cover the films with liquid pet- 

 rolatum or petrolatum (vaseline) and keep them away from 

 the light. The alcohol is used to prevent the formation of a ground- 

 glass deposit which sometimes occurs after the use of xylol alone ; 

 and if a cold slide is washed with xylol and alcohol enough water 

 may condense on it to dilute the alcohol and partly decolorize 

 the film. Where one wishes to avoid the use of cedar oil for 

 immersion, liquid petrolatum, heavy (U.S. P. IX) will prove to be 

 a fair substitute. In that case the same medium will serve for 

 examination and preservative. 



Advantages of the thick film method, especially for malaria 

 surveys, have been recognized by all who have given it a fair trial. 

 An assistant may be easily taught to collect good specimens, and 

 the method has been widely and successfully used in field work. 

 Much time is saved in the examination of specimens. When para- 

 sites are at all numerous they are usually picked up in the first 

 thick-film field; when they are rare, they are often detected in the 

 thick film when they might have been missed in a thin or found 

 only after a long search. The chief purpose of the thick film is, of 

 course, the diagnosis of malaria rather than the study of the 

 characteristics of malaria parasites, a purpose for which the 

 thin film is more suitable. 



Recognition of parasites in the thick film. The difficulty of 

 learning to recognize parasites in the thick films has been over- 

 estimated. Examiners familiar with the appearance of parasites 



