A sextant is a navigational device. It uses a small telescope and a series of mirrors and shades to help determine a ship's position on the earth's surface.
Lieutenant Eric Gault Finley used this sextant when he served in the corvette HMCS Shawinigan. By measuring angles between celestial objects, like the sun, and the horizon, the sextant allows a user to determine longitude (east-west position). To protect against damage that might affect the instrument's accuracy, sextants were stored when not in use in protective cases like the one seen here.