66 HOW TO WORK 



They may be purchased of Mr. Bailey, of Fenchurch Street, at 6d. 

 a-dozen. 



132. Round Cells. My friend and colleague, Dr. Guy, has lately 

 proposed a form of cell which possesses many advantages over those 

 in common use. These are circular, and may be made of bone, 

 metal, gutta percha, or glass, of various depths, and to suit trans- 

 parent and opaque objects. Several forms have been made. They 

 are all of the same external diameter, and are made to fit into a rim 

 of equal size in a flat plate of wood, or metal, which can be placed 

 in the field of the microscope. A small cabinet will contain many 

 more preparations mounted in this manner than on the ordinary 

 slips of glass. Dr. Guy has had some circular labels printed for 

 these cells upon which the names of the preparations may be written, 

 and as these are of different colours the various microscopic objects 

 can be readily classified. 



133. Troughs for Examining Zoophytes. These are deep but 

 very narrow glass cells, the two surfaces consisting of very thin glass, 

 so that the higher powers may be brought sufficiently close to the 

 objects. The opening is above, so that the cell with living animals 

 within may be placed upon the stage of the microscope, when the 

 instrument is inclined, without any fluid escaping. It is convenient 

 to have a glass partition in these troughs, by means of which objects 

 may be placed in different parts of the cell. A convenient size is 

 three inches long, an inch and a half deep, and a quarter of an inch 

 in width. 



134. Animalcule Cage. Another very convenient form of cell is 

 the one called animalcule cage, pi. VII, fig.2S, pi. XVIII, fig. 112. 

 By means of its sliding cover a stratum of fluid of any required 

 thickness can be obtained, and small living animals can be con- 

 veniently fixed in positions suitable for observation. For the 

 examination of deposits in fluids this form of cell is also very 

 convenient. 



135. Growing Cells. In many investigations upon the lower forms 

 of vegetables and animals which live in water, it is necessary to watch 

 the same specimen for a considerable time. Some plan must there- 

 fore be adopted by which the living object can be freely supplied 

 with fresh water and air. Numerous forms of growing cells have been 

 proposed, but I shall only refer to two or three which seem to me to 

 be most advantageous. The following brief description of an im- 

 proved growing-trough by its ingenious deviser, Prof. Smith, of Kenyon 

 College, United States, is taken from Silliman's American Journal 

 of Science, September, 1865, also Mag. Nat. Hist., vol. XVI, 1865. 

 The whole slide is a trifle more than one-eighth of an inch in thick- 



