In that case _Bartley’s Warm Stage_ (Fig. 224) is recommended. There are other forms of warm stages in use, many of an inexpensive kind and readily adaptable to any stage. Bartley’s has proved useful; it consists of a vessel, _E_, three parts filled with water and supported on a ring stand. This may be kept at any temperature by the small spirit-lamp, _C_; a syphon tube _d_ conveys the warm water along _f_, and through the bent tubing which surrounds the object under observation on the stage, _D_, and then passes off through the open end, _C_, into the receptacle, _B_, placed to receive the overflow. Steam can be used for heating, or iced water for observing the effects of cold upon the organism. A simple form of warm stage may be made of an oblong copper plate, two inches long by one wide, from one side of which a rod of the same material projects. The plate has a round aperture, the centre half an inch in diameter, and is fastened to an ordinary slide with sealing-wax. The drop or object to be examined is placed on a large-sized cover-glass and covered over with a smaller one. Olive oil or vaseline is painted round the edge of the smaller one to prevent evaporation, and the preparation is placed over the aperture in the plate. The slide bearing the copper plate is clamped to the stage of the microscope. The flame of the spirit-lamp is applied to the extremity of the rod, and the heat is conducted to the plate and thence transmitted to the specimen. In order that the temperature of the copper plate may be approximately that of the body, the lamp is so adjusted that a fragment of cacao butter and wax placed close to the preparation is melted. Professors Stricker and Schäfer have constructed warm stages for accurate observations, and which fully answer every purpose. [Illustration: Fig. 225.--Stricker’s Warm Stage.] _Stricker’s Stage_ (Fig. 225) consists of a rectangular box with a central opening, _C_, permitting the passage of light through the specimen under examination. The water makes its exit and entrance at the side tubes _B B′_, and the temperature is indicated by a thermometer in front. In this apparatus either warm or cold water can be continuously used. [Illustration: Fig. 226.--Schäfer’s Warm Stage.] _Schäfer’s apparatus_ (Fig. 226) consists of a vessel filled with water (seen near the stage) which has been first boiled to expel the air, and then heated by means of a gas flame. The warm water ascends the india-rubber tubing to the brass box on the stage. The box is pierced by a tubular aperture to admit light to the object, and has an exit tube by which the cooled water from the stage returns by another piece of tubing to be reheated by the gas flame. There is a gas-regulator, by means of which any temperature can be maintained. Methods of Preparing, Hardening, Staining and Section Cutting.