These small glass tubes are infrared photocells built circa 1946 by Robert Cashman of Northwestern University. These tubes are coated on part of their interior faces with a very thin evaporated layer of lead sulfide (PbS). The conductivity of the coating is modified by impinging infrared photons. This change in conductivity is a direct measure of the intensity of the infrared radiation when amplified by appropriate electronic circuits. Detectors such as this were developed for military use duirng World War II, and as a result were classified. A few astronomers were keenly aware of their existence, and some gained access to the technology in the 1940s. In particular they were used for studies of the infrared spectra of planetary bodies by Cashman and his collaborator Gerard Kuiper at nearby Yerkes Observatory. The set of detectors in the collection was donated to NASM in 1994 by Dale P. Cruikshank, a student of Kuiper.