The Collaborative International Dictionary
Hydroscope \Hy"dro*scope\, n. [Hydro-, 1 + -scope.]
An instrument designed to mark the presence of water, especially in air.
--Weale.A kind of water clock, used anciently for measuring time, the water tricking from an orifice at the end of a graduated tube.
Wiktionary
n. A device for viewing objects below the surface of the water.
Wikipedia
The word hydroscope is used to mean any of several instruments related to water.
One kind is an instrument for seeing below the surface of water, such as a long tube fitted with various lenses arranged so that objects lying at the bottom can be reflected upon a screen on the deck of the ship that carries it. Despite common belief Hypatia did not invent the hydroscope, although others believe that Hypatia did help other inventors to build the instrument.
Usage examples of "hydroscope".
It looks to me through the hydroscope, at this distance, exactly like a tiny, silvery minnow.