364 RESEARCH IN PROTOZOOLOGY 



to spend a good deal of time on a film, but here it is usually 

 possible to get a new specimen taken at a time when parasites 

 may appear in larger numbers. 



The question of time of examinations is usually most important 

 in connection with malaria-parasite surveys. It is difficult to 

 establish a standard time since the skill of individual examiners 

 varies so greatly. We recommend that each examiner determine 

 for himself the amount of error incident on shortening the time 

 devoted to thick films apparently negative. Select a batch of slides 

 where a fair percentage, say twenty per cent or more, of positives 

 may be expected. Note the amount of time required to find the 

 first parasite in each positive, and continue the examination of 

 negatives for ten minutes. It then becomes possible to estimate 

 the error which would have been incurred had the time been 

 shortened. The skilled examiner will probably find that a very small 

 percentage will be missed if the examining time be limited to 

 three or even two minutes. The experiment- may be worked out in 

 terms of the number of fields examined instead of the time devoted 

 to the search. When basophilia or other evidence of anemia, or 

 suspected remnants of parasites are found in a preparation, the 

 examination is prolonged. 



If a survey of a given group of persons be repeated after a few 

 days one usually obtains new positives among former negatives ; 

 the best single examination can disclose only a fraction of the 

 persons actually infected. But a single survey gives results of 

 much comparative value, especially if the examiner follows the 

 golden rule of recording as negative all doubtful specimens. The 

 examiner is then sure that the population contains at least the 

 percentage positive which he has obtained, and if he has ascer- 

 tained his examination-time-error and has adopted a standard 

 time, he will obtain results reliable for comparing the malaria 

 rate of one population with that of another. It is of questionable 

 utility to attempt to prescribe an examination-time applicable to all 

 examiners and all circumstances. 



Much time can be saved in collecting blood specimens if the 

 slides, boxes, and other apparatus are placed before the collector 

 in convenient order. The collector sits alongside a table, near the 

 end. His right side should be nearest the table. The slide box is 

 set upright a little to the left and in front of the collector. The 

 cover of the slide box is placed next in order to the right, and the 



