APPENDIX. 327 



in exact focus is seen withia a circular space, called the 

 field of mew. 



330. Many objects, as pollen grains, spores, hairs, etc., 

 require no special manipulation preparatory to their exami- 

 nation. They need simply to be placed on a narrow slip 

 of glass, which should be perfectly clear, called a slide. 

 The commonest size of glass-slides, js one by three inches. 

 A preferable form is that now coming into extended use ; 

 it has a size of twenty-five by forty-five millimetres. 

 Over the object must invariably be placed a very thin 

 piece of glass, called the cover-glass. As a rule, objects 

 must be placed or mounted (as it is called) in water, 

 alcohol, glycerine, or balsam. Other reagents, as all 

 these substances are called, are often needed, as potash, 

 acetic and other acids, iodine, hsematoxylin, and other stain- 

 ing reagents. The use of alcohol in preparing objects for 

 examination is important, as will early be learned by ex- 

 perience. It is invaluable for driving put bubbles of air 

 that are entangled in the tissues or other objects to be 

 examined. These air bubbles are the great pest of the 

 mioroscopist,' and even alcohol will not always expel them 

 entirely. They appear as round, transparent bodies, bor- 

 dered by a dark line, and will doubtless be seen, even if not 

 recognized, in the first specimen mounted by the amateur. 

 The potash solution tends to clear specimens that are dark, 

 and is indispensable when judiciously used. Iodine has 

 many uses, and should be invariably at hand. Glycerine 

 is one of the most convenient and valuable of the mounting 

 fluids. All these and other reagents are supplied in small 

 bottles by those who deal in microscopes and microscopists' 

 supplies. 



331. The cutting or making thin sections of tissues 



