286 MANIPULATION. 



The Concave CeW.— This consists of a slide of plate-glass, 

 rather thicker than ordinary, in the centre of which a concave 

 cell or pond has been ground out, and either left in the rough 

 state or polished ; it may be either of the form represented in 

 fig. 178 or fig. 179, and the method of mounting objects in it 

 p. ^yg is the same as in the flat 



cell; a thin layer of the 

 gold-size or other cement 

 is to be placed around the 

 margin of the cover as in 

 the case of the flat cell, and 

 the excess wiped off with 



the finger. The fluid is to 



Fig- 179. u 1 J • ..1. 



° be placed in the concavity, 



and the cover dropped on as in the preceding descriptions, but 

 the cover should be so much larger than the cell, as to leave a 

 margin of one-eighth of an inch around it. This form of cell, 

 when polished, does very well for many objects where accurate 

 definition of the surface only is required ; and, when unpol- 

 ished, is most useful for thin opaque objects, such as pieces 

 of injection, leaves of plants, &c., &c., that are too thick to be 

 mounted between two flat pieces of glass. 



White Lead Cell. — A very convenient and durable form of 

 cell may be made by spreading some old white lead, that has 

 been ground in oil, on one of the slides that measure three 

 inches by one, taking care to leave an aperture or pond in 

 the centre a little larger than the object; the lead may be laid 

 on of a thickness equal to that of the object. Spirit and water, 

 or other fluid, is to be placed in the pond, and the object de- 

 posited in it ; the thin glass cover, which should be as large as 

 convenient, is to be put on obliquely, and pressed firmly down 

 into the white lead, taking care to exclude all air bubbles. 

 The author has objects in his possession, quite perfect, which 

 were mounted eleven years ago. Mr. William Valentine, who 

 has adopted this plan for many years with complete success, 

 uses a little trowel of box or other hard wood of the shape 

 represented by fig. 180 to plaster down the white lead and 

 press the glass, and a more convenient instrument for the 

 purpose cannot be devised. When Mr. Valentine first com- 



