and sterilizing flames. One line of research is to subject organisms to the influence of different densities in the surrounding medium. For fluids there are basins containing different concentrations of salts, brackish water, etc., with an areometer for measuring the specific gravity of each of the various liquids. For gases there are hand air pumps and means of attachment to the supply pipes of the large electric air pump located in the cellar. For changing the normal influence of gravitation there are clinostats regulated by clockwork and centrifuges with contact plugs for tapping the electric power wherever needed.
Two dark rooms (Fig. 11) are fitted for studying the influence of light. Both open into a dark corridor from which a triangular door
leads into an anteroom lighted by a red dark-room light. The larger dark room serves for investigations upon the influence of definitely determined lights. Hung up at one end is an arc lamp in which different colored carbons can be used. The elongated form of the room makes possible experiments in which the organisms are placed at different distances from the light source. For investigating the influence of variously colored surroundings, small vivaria are placed in lighted rooms with colored glass covers for direct light, or colored wall coverings, for reflected light. A special apparatus has been constructed for the study of under and over lighting of fishes and amphibia (Fig. 12). For over lighting the aquarium is covered with an enclosing box open above (o) and for under lighting the box is closed above (u). For in-