METHOD OF PREPARING AND EXAMINING THICK-FILMS 363 



films and may assume more rounded forms, which may simulate the 

 schizonts of benign tertian or quartan. The more compact pigment 

 of crescents, the deeper staining of the cytoplasm, and, in some 

 cases, a pink remnant of the host cell will aid in identifying 

 doubtful forms. At the margin of thick films crescents dry more 

 rapidly and are more apt to retain their typical form. Except 

 crescents, larger parasites of estivo-autumnal are very rare in the 

 peripheral blood. 



Mixed infections are commonly overlooked in any kind of film 

 unless the parasites of the two or more species occur in some 

 very characteristic form. Thin films may show the specific charac- 

 ters more clearly, but, more often than thick films probably, fail 

 to disclose a mixed infection, since fewer parasites are commonly 

 examined. In any case it is well to continue the search after the 

 detection of the first parasite. In any sort of preparation where 

 only a few young rings are present, it is sometimes impossible to 

 identify the species. 



We generally use the thin film for identifying and estimating 

 the number of gametocytes of benign tertian and quartan. In the 

 case of crescents, the thick film is more suitable. 



Minhnmn equipment for examining thick films. Any standard 

 microscope fitted with a good mechanical stage is suitable. Ex- 

 pensive apochromatic objectives are not necessary, but much 

 greater flatness of field and increased definition may be obtained 

 by the use of a good fiuorite objective, such as the Bausch and 

 Lomb Xo. 1043. It is a good plan to use an eyepiece of relatively 

 low magnification, such as a 5X, as a searcher, but also to have 

 a good 7.5X eyepiece to give higher magnification for examina- 

 tion of doubtful parasite appearances. The usual small substage 

 lamp is inadequate for illumination ; the chalet type of lamp 

 fitted with lOO-watt electric bulb and daylight glass window is to 

 be preferred. 



Length of time to devote to thick films apparently negative. 

 It is commonly recommended that fifteen to twenty minutes be 

 devoted to a thin film before it is declared negative, and five 

 minutes to the thick film. In either case the time spent on ap- 

 parently negative specimens must vary with circumstances. When, 

 for example, the sole purpose is to find a crescent carrier suitable 

 for mosquito-infection experiments, a fraction of a minute will 

 suffice for the thick film. In a clinical case it may be necessary 



