42 OF THE CHARACTERISTICS OF THE DIATOMACEuE. 
glass.* The next step is to give an almost imperceptible 
inclination to the covering glass by gently pressing one of 
the edges with a fine mounted needle. This causes a tiny 
stream to flow under the glass, by which means the plants 
are put in motion, and begin to roll over. Meanwhile the 
object under examination must not be allowed to escape 
from the operator’s view (a mishap which will inevitably 
occur if the covering glass be too roughly handled), as it 
is often lost difficult to catch sight of it again. Another 
reason for exercising great care in the use of the needle is, 
that in very flat species (such as Himantidium) the hoop 
is so narrow as to form a mere suture. Consequently the 
frustule, as it turns over, assumes an erect position for a 
single second only, immediately resuming its former posi- 
tion as the stream of water flows back; so that, unless 
extreme care be used, the observer will scarcely ever 
succeed in getting a front view. 
It happens occasionally that the plants themselves present 
both valve and hoop to view, or that they rest against 
some foreign body in such a way as to show them, without 
any special manipulation ; but these must be looked upon 
as happy accidents. 
If it is desired to make a drawing of these delicate 
organisms, and I would strongly recommend the collector 
to do so, this may be accomplished by means of the camera 
lucida, or Sommering’s mirror or steel disk. For a de- 
scription of these instruments, and the method of using 
them, I must refer the reader to works more peculiarly de- 
voted to the microscope.^ 
The markings on the silicious valves of the Diatomacese 
* The thicker this covering glass is (in reason), the more conve- 
nient is it for the operator ; because very thin glasses are difficult to 
manage, and the water is apt to overflow their edges, and dim the 
surface. Of course the positive thickness of the glass must bo 
decided by the powers of the objective in use at the time. 
f Micrographic Dictionary, Introduction, p. xix. ; or Beale’s ‘ How 
to Work with the Microscope,’ p. 20. 
