An apparatus free from this fault is shown in Fig. 13. It consists essentially of a counterbalanced lever A, to which the upper jaw J is rigidly attached. By means of a sliding weight W, any desired load, from zero upwards, may be put on this jaw. The lever carries a very delicate spirit level, which allows it to be set accurately horizontal in every experiment. The lower jaw is movable vertically between parallel guides, and its position is controlled by the screw-wheel S which bears a divided circle on its circumference. The position of this wheel, and therefore of the lower jaw, may be accurately read on the vernier _v_. In every experiment it is adjusted so as to make the upper lever accurately horizontal. [Illustration: Fig. 13.--Improved Apparatus for Measuring the Degree of Falling. (Sand.)] CHAPTER IV. THE BACTERIOLOGY OF THE BATE. “Omne vivum ex ovo.”--HARVEY (1578–1675). When a drop of liquid from a puer wheel in use is examined under the microscope[73] with 1/12 o.i. objective, it is seen to be swarming with bacteria. [73] A good microscope, with 1/12 oil-immersion objective and Abbe condenser, is necessary for bacteriological work. The new dark-ground condenser made by James Swift and Son, London, is extremely useful for examining living bacteria under high magnifications. The majority are short rods (bacilli), but other forms, cocci and spirilli, are seen in lesser numbers. Most of these bacteria move briskly in the liquid; as the temperature of the slide sinks, their movements become slower, and finally cease. The illustration, Fig. 14, shows the various forms of bacteria observed by the author in puer liquors × 1000 diam. [Illustration: Fig. 14.--Various Forms of Bacteria in Puer Liquors.] The living bacteria are best examined in a drop culture in the following manner. A clean cover-glass, of the proper thickness for the objective to be used, is laid upon a black glass plate. With a platinum loop, previously heated to redness in the flame, a drop of sterile physiological salt solution (0·6 to 0·75 per cent.) or sterile broth is placed in the centre of the cover-glass. With a platinum needle a minute quantity of the puer liquor is stirred into the drop. A slide with a depression in the centre is taken, the edge of the depression painted round with vaseline, and pressed over the cover-glass, so that the drop is exactly central. If the whole be now turned smartly over, the drop will hang central in the hollow space. If the ring of vaseline is continuous, and the cover well pressed down, the drop is preserved from evaporation, and the bacteria may be examined in their natural condition--best on the edge of the drop. For illuminating the drop culture, the concave mirror is used, and a small diaphragm without condenser; whereas, for stained preparations, the flat mirror is used in conjunction with the Abbe condenser.