DIATOMS. O48 
more recently it was an accomplishment to ‘‘ dot” the valve 
of an Angulatum. Indeed to compass the latter feat, 
with lenses made before the fifties, required more than 
average manipulative skill. The magnifying power used 
in those days rarely exceeded four to six hundred diameters. 
Rabenhorst for his Susswasser-diatomaceen used a power of 
only three hundred. And, as lately as 1890, the magnify- 
ing power used for Wolle’s plates was uniformly five 
hundred. But in 1865 Powell and Leland made a fiftieth 
objective having a magnifying power of nine thousand 
diameters. That, however, was the product of special 
skill, and required for its use the rare ability of both Mr. 
Powell and Dr. Beale. Still immersion lenses of moderate 
price greater working distance, wide angular aperture, 
special light condensers, and manifold greater magnifying 
power soon came into general use. The fragment of an 
angulatum frustule under modern high power objectives 
can now readily be magnified five thousand diameters or 
more ; and what formerly seemed only fine hatched lines, 
according to illumination used, are now resolved into rows 
of pearly dots ten to the inch, or hexagonal depressions of 
like size. For under the microscope things are not always 
as they seem, and errors of interpretation grow more likely 
with increased amplification. Hence itis that the markings 
of the highly magnified frustule of the angulatum are 
described by some as raised globules, by some as areolate 
depressions, and by others as apertures in the frustule. 
Many species of Diatoms possess the power of locomo- 
tion ; a faculty which arrests the attention of observers, 
and which led Ehrenberg to call these organisms animals. 
In the Naviculz this power is highly developed, especially 
in the minute frustules of the species Navicula Veneta. 
Dr. Donkin, an authority, well described their motion 
thus: ‘“‘ Placed under the microscope several individuals 
may be seen directing their course simultaneously, and as it 
were voluntarily toward any extraneous object near them. 
