_The Compressorium_ is used for similar purposes. By a graduated pressure the fluid is _thinned out_ and a higher power can be employed for the examination of the object. Ross’s early compressorium consists of a plate of brass about three inches long, having in its centre a circle of glass like the bottom of the live-box. This piece of glass is set in a frame, _B_, which slides in and out so that it can be removed for the convenience of preparing any object upon it--under water if desirable. The upper movable part, _D_, is attached to a screw-motion at _C_; and at one end of the brass plate, _A_, which forms the bed of the instrument, is an upright piece of brass grooved so as to receive a vertical plate, to which a downward motion is given by a single fine screw, surrounded by a spiral spring, which elevates the plate as soon as the screw-pressure is removed. [Illustration: Fig. 210.--Beck’s Parallel-plate Compressor.] _Beck’s Parallel-plate Compressor_ (Fig. 210) affords a more exact means of regulating the pressure, and can be used for a variety of purposes. It is also easily cleaned. [Illustration: Fig. 211.--Rousselet’s Compressorium.] _Rousselet’s Compressorium_ (Fig. 211) is a very effective form for general use. It is so arranged that the student has perfect control over the pressure to which the specimen should be subjected. The cover-glass is large in comparison with that beneath; being bevelled causes evaporation to go on very slowly while the pressure between the two glass surfaces is kept perfectly parallel. [Illustration: Fig. 212.--Botterill’s Live-trough.] _Botterill’s Live-trough_ (Fig. 212) consists of two brass plates screwed together by binding screws, and holding between them two plates of thin glass, which are maintained at a proper distance by inserting a semicircular flat disc of india-rubber. [Illustration: Fig. 213.--Glass Trough.] Glass troughs for chara and polypes (a sectional view of one shown at Fig. 213) are made of three pieces of glass, the bottom being a thick strip, and the front (_a_) of thinner glass than the back (_b_); the whole is cemented together with Jeffery’s marine-glue. The method adopted for confining objects near the front glass varies according to circumstances. The most convenient is to place in the trough a piece of glass wide enough to stand across diagonally, as at _c_; then, if the object be heavier than water, it will sink until stopped by the glass plate. At other times, when used to view chara, the diagonal plate may be made to press it close to the front by means of a wedge of glass or cork. When using the trough the microscope should be placed in a nearly horizontal position. [Illustration: Fig. 214.--Weber’s Slip with Convex Cell for use as a Live-trough.] [Illustration: Fig. 215.--Current-slide Live-cell.]