Cells for viewing living objects, and watching their movements, take many forms, usually determined by the makers for the purposes they are required to serve. The smaller glass troughs (Figs. 216, 216_a_) are made for examining the small infusoria, rotifers, &c., some of which take special forms, as the double or divided trough (Fig. 217) intended for viewing the circulation of the blood in the tail of a small fish, and at the same time keep up a supply of water and air. [Illustration: Fig. 216.] [Illustration: Fig. 216_a_.] The Frog-plate consists of a strip of plate-glass, or wood, pierced with holes on either side, through which tapes are passed to secure the frog in its place. At the extreme end is a shallow glass trough, made to hold a sufficient quantity of water to keep the web of the foot moist while under examination. In this way a continuous view of the circulation of the blood of the animal is obtained. [Illustration: Fig. 217.] _Growing Cells_ have received more attention from those who devote attention to the lower forms of life, the construction of which, for the purpose of maintaining a continuous supply of fresh water to objects under observation, and for sustaining their vital energy for a long period, is of some importance. The employment of live-cells is resorted to by microscopists, as doubtless there is much to be discovered concerning the metamorphoses which some of the lower micro-organisms, both of plant and animal life, pass through. [Illustration: Fig. 218.--Frog-plate.] Holman’s life slide consists of a 3 × 1 inch glass slide, with a deep oval cavity in the middle to receive the specimen for observation. A shallow oval is ground and polished around the deep cavity, forming a bevel. From this bevel a fine cut extends, to furnish fresh air to the living low forms of life which invariably seek the bevelled edge of the cavity, thus bringing them within reach of the highest powers. He also contrived a convenient form of “moist chamber,” or animalcule-cage (Fig. 220), for the purpose of studying the growth of minute organisms, without in any way disturbing them for a lengthened period. This is also found useful as a dry chamber for holding minute insects. [Illustration: Fig. 219.--Holman’s Life Slide. Full size.] [Illustration: Fig. 220.--Holman’s Moist Chamber.] _Zentmayer’s Holman Syphon Slide_ is used either as a hot or cold water cell. It should be deep enough to hold a small fish or newt, and retain it without any undue pressure. When in use it is only necessary to place the animal into it (as shown in Fig. 221), with some water, and secure it with a glass cover; then immerse the upper tube in a jar of water, while another, at a lower level, maintains a current. When the slide is on the stage of the microscope, one jar should stand on a lower level than the other, the slide being made the highest part of the syphon. The pressure of the atmosphere is sufficient to keep the cover-glass in its place.