270 FIRST FORMS OF VEGETATION. 
The spectrum in question is a very characteristic 
one, and cannot be mistaken. It exhibits a very 
black, narrowish band in the extreme red, reading at 
the lower edge, which appears to be remarkably 
constant, about 2 of Mr. Sorby’s scale. 
The forms which the diatomacee assume are 
exceedingly varied and beautiful. Most of them, 
as already mentioned, are invested with a very 
thin transparent glass-like pellicle, engraved with 
median lines and transverse striz, the patterns of 
which are wonderfully constant in the same species, 
and afford admirable tests for the general excel- 
lence of the object-glass of the microscope; the 
distance between the different markings being 
often the spgoath part of an inch, and some, it is 
even said, being only the zsgyaath of an inch sepa- 
rate, requiring for their distinct determination a 
magnifying power of twelve hundred diameters, 
and the aid of oblique light. What appear as 
strize with low powers, assume the form of monili- 
form or pearl necklace-like markings when exam- 
ined with high powers. Their silicious investment 
has cellulose for its base. The silex is infiltrated 
to a variable extent in the different families, and 
the mode of its deposition can to a certain extent 
be ascertained by examination with polarized light. 
In order that the striz or markings may be clearly 
seen it is necessary that the valves or frustules 
