LABOEATOEY EESEAECH, ETC. 259 



they have been stained by the process we are about to 

 mention. 



A preparation ought first to be examined under a 

 low magnifying power (from 50 to 100 diameters), so 

 as to study the topography of the object, and ascertain 

 the points at which the colonies of microbes may be 

 sought amid the tissues of a section, or of the matters 

 in suspension in the liquid. 



We should then go on to a higher magnifying 

 power (for example, to from 500 to 700 diameters), 

 making use of the simple light of the mirror ; and we 

 should ultimately come to the highest magnifying 

 powers (from 1000 to 1500 diameters), using immer- 

 sion-lenses and the condenser. 



Instruments, Microtome. — The instruments for fine 

 dissection are those commonly used in histology. In 

 addition, needles of glass and platinum are necessary, 

 and thin spatulas of nickel to convey the sections, etc. 



The ordinary razor, which serves for hand sections, 

 will not do for the thin, wide sections necessary for 

 the discovery of bacteria. In this case a microtome 

 must be used, an instrument for making thin sections, 

 for which purpose those of Thoma or V^rick are the 

 best. Sometimes the object to be examined is 

 hardened by freezing it with ether spray, since this 

 makes it possible to cut thin sections by hand. This 

 is Jung's process. 



Mon - staining Liquid Reagents. — Acids, bases, 

 alcohol, oil of aniline, and other essences serve to 



