INFUSORIA. 
23 
is forming in some other part; one portion is contracting, 
another enlarging; so that the only idea that can be 
given of its shape, is by comparing it to the figure of a 
country upon a map, which is perpetually transformed 
into that of some other country. 
These are but a few examples of the variety of form 
which this curious class of animals presents to us : it re- 
mains now only to give, to those who may be desirous of 
examining them for themselves, a few instructions to aid 
their researches. A good microscope is, of course, indis- 
pensable ; if the study is intended to be pursued scienti- 
fically, a compound achromatic, with magnifying powers 
ranging from fifty to five hundred diameters, must be 
employed ; but very much entertainment, and a good 
idea of many of the forms, may be obtained with an 
ordinary simple microscope of three powers. The pene- 
trating and defining properties of a microscope are of more 
importance than mere enlargement. 
If we were asked where specimens are to be obtained, 
we might reply, almost everywhere, provided water be 
present : but even in the air they have been lately 
detected by the eminent Prussian professor. In ditches 
and ponds, in the trenches of meadows, in the ruts of 
highroads, in marshes, in lakes and rivers, in estuaries, and 
even in the sea, various species may at almost all times be 
found. Stagnant waters, especially such as aro covered 
with a thick scum, contain some species in immense 
abundance, but. these arc generally of the simplest struc- 
ture : little hollows in boggy ground, especially where a 
red sediment betokens the presence of oxide of iron, fre- 
quently contain species of great rarity and beauty ; but 
