Definition of water telescope in US English:

water telescope

noun

  • 1Astronomy a telescope whose tube is filled with water, used especially to investigate aberration of light and the propagation of light in different media (now historical and rare).

  • 2An instrument used to make observations beneath the surface of water, consisting of a container or tube with a transparent bottom.

Origin

Late 18th century; earliest use found in Philosophical Transactions of the Royal Society of London.