OF MICROSCOPIC OBJECTS. 29 



having first slightly damped the ends of the fingers employed 

 to obtain firm hold. When the dirt is very obstinate, 

 breathing upon the glass greatly facilitates its removal, and 

 the sense of touch becomes so delicate that the breakage is 

 inconsiderable ; but this method cannot be recommended to 

 novices, as nothing but time spent in delicate manipulation 

 can give the sensitiveness required. 



It has been before mentioned that ordinary glass slides are 

 sometimes worthless, especially for fine objects^ from having 

 a rough surface, which presents a dusty appearance under 

 the microscope. This imperfection exists in some thin glass 

 also, and is irremediable ; so that it is useless to attempt to 

 cleanse it ; nevertheless, care should be taken not to mistake 

 dirt for this roughness, lest good glass be laid aside for a 

 fault which docs not really belong to it. 



When anv object which it is devsired to mount is of con- 

 siderable thickness, or will not bear pressure, it is evident 

 that a wall must be raised around it to support the thin 

 glass this is usually termed a " cell." There are various 

 descriptions of these, according to the class of objects they 

 are required to protect ; and here may be given a description 

 of those which are most generally used in mounting "dry" 

 objects, leaving those required for the preservation of liquids 

 until we come to the consideration of that mode of mounting. 

 Many have used the following slides. Two pieces of hard 

 wood of the usual size (3 in. by 1 in.), not exceeding one- 

 sixteenth of an inch in thickness, are taken, and a hole is 

 then drilled in the middle of one of these of the size required. 

 The two pieces are then united by glue or other cement, 

 and left under pressure until thoroughly dry, when the cell 

 is fit for use. Others substitute cardboard for the lower 

 piece of wood, which is less tedious, and is strong enough 

 for every purpose. This class of " cell " is, of couise, fitted 

 for opaque objects only where no light is required from 

 below; and as almost all such are better seen when on a dark 

 background, it is usual to fix a small piece of black paper 

 at the bottom of the cell upon which to place them. For 



