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The NIST electronic kilogram has had three published results with successively smaller uncertainties. In the present design, there are several measurement components that have uncertainties that are difficult to reduce further. These limiting uncertainties are discussed.

Research in the microbiology of the upper air at the Massachusetts Institute of Technology was inaugurated in 1932. During the past five years, airplane collections of microorganisms and other air-borne materials have been made, in the course of more than two hundred separate flights in the vicinity of Boston. The first instrument used at the Massachusetts Institute of Technology was described by one of us (Proctor, 1934) together with certain of the findings made previous to that time.Workers in this and other countries have been engaged in somewhat similar researches which have added a number of different devices capable of use for such collections, both from airplanes and balloons. Among the devices of more recent origin are those described by Lindbergh andMeier (1935), andRogers andMeier (1936) in the United States, and Van Overeem (1936) in Holland.The present paper is concerned with modifications in the bioaercollector and in the laboratory technique involved in its use which minimize the opportunity for contamination during collection and examination. The modifications also permit photographic records under aseptic conditions of the non-viable particulate matter which comprises the major portion of the collections, thus allowing subsequent culture of the living material collected.