The apparatus can be made cheaply in the lab. if used with an existing hand spectroscope. It consists of a cup, 61/4 ins. long, and a plunger 7 ins. long with the spectroscope above and a light-source below. The parts are mounted in glass sleeves which slide in one another and in a wooden housing which also contains the electric bulb. Only 5 c.c. of soln. are required for varying the depth from 1 to 125 mm.