OF THE CHARACTERISTICS OF THE DIATOMACEjE. 
45 
long, the markings lose their sharpness to a great extent,, 
owing to a kind of glazing operation, which follows on the 
melting of the alkalies contained in the water. On this 
account distilled water should be employed, so as to avoid 
the presence of calcareous salts as far as possible. 
After this preparation, the frustules may be placed under 
the microscope, where a most astonishing sight awaits the 
eye. So wonderfully minute, however, are the markings 
brought out by the incineration of the vegetable matter, 
that, except in a few rare instances, they escape detection 
altogether when the transmitted light is direct ; that is to 
say, when the rays falling upon the object are parallel to 
the axis of the instrument. In most kinds of Diatomacese 
the light must be directed obliquely to the lines of stria*, 
before their minute structure becomes visible even with 
the highest powers. Oblique light, it may be observed, is 
obtained by thrusting the mirror to one side of the axis 
of the microscope. The diaphragm should be removed, to 
allow free passage to the rays, when the latter will fall, so 
to speak, upon the edges of the lines, and render them 
visible. If this proceeding is skilfully managed, the whole 
system of lines, of which some species possess several, 
running in different directions, is brought under the eye of 
the observer. 
It is well here to remind the student, that, in order to 
sharpen his experience and to accustom himself to the 
detection of these minute points, he should provide himself 
with what are commonly termed { test objects,’ of which the 
Diatomaceous family offer some excellent examples. For 
the lower powers, those magnifying from 200 to 220 times 
linear,* Pleurosigma attenuatum forms a good test; for 
. * ‘ When one dimension only of an object is taken into account, 
viz. the breadth or diameter — and this is the ordinary manner in 
which the magnifying power is taken— objects are then said to be 
magnified so many diameters, or so many times linear. But objects 
are really as much magnified in the other dimension, or in their 
entire surface; so that the true expression of their amplification 
