1) Optical device 
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 so arranged 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 i...
instrument for viewing under water
• (n.) An instrument designed to mark the presence of water, especially in air. • (n.) A kind of water clock, used anciently for measuring time, the water tricking from an orifice at the end of a graduated tube.
Hy'dro·scope noun [ Hydro- , 1 + -scope .] 1. An instrument designed to mark the presence of water, especially in air. Weale. 2. A kind of water clock, used anciently for measuring time, the water tricking from an orifice at the end of a graduated tube. 
hydroscope, hydroscopic An optical device used for viewing objects far below the surface of water.
an optical device for viewing objects below the surface of water.