From AMS Glossary
A special form of the aspiration psychrometer, developed by Assmann.
Two mercury thermometers, mounted vertically side by side in a chromium- or nickel-plated polished metal frame, are connected by ducts to an aspirator. Each thermometer is located inside a pair of coaxial metal tubes, highly polished inside and out, that screen the bulbs from solar radiation.
Middleton, W. E. K., and A. F. Spilhaus 1953. Meteorological Instruments. 3rd ed., rev., . Univ. of Toronto Press, . 102–104.