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POPULAR SCIENCE REVIEW. 
and striae, rectilineal and oblique under low powers, and of 
hexagons and other fancies under high powers, I saw what 
really does exist, viz. a series of beautiful hemispheres placed 
in their due order on the silicious tissue of the valve. If the 
little hemisphere under examination were really the size that 
our powers show it, it would be seen by unassisted vision, and 
we should smile at a supposed necessity of forming its shadow 
by two sources of light, just as an artist would smile if he were 
advised to have two windows in his studio at right angles to 
'each other for the more artistic illumination of his sitter. The 
moon, as shown by the sun’s illumination, is a fair illustration 
of diatom-illumination. Light, virtually parallel, falling obliquely 
on one side only of its mountains and craters produces natural 
light and shade. Any other arrangement would fail, and for 
this reason pencils of light placed at right angles or otherwise 
in the kettledrum lead to illusions. The kettledrum, however, 
with one aperture properly placed, is still a serviceable condenser, 
and brings out the hemispheres remarkably well. This con- 
denser may be looked upon as monochromatic, since any one of 
the divisional colours of the spectrum may be used ad libitum. 
Mr. Slack very naturally prefers Ross’s four-tenths achromatic 
condenser with a single radial aperture. Still refracted light 
has not the power and purity of reflected light, and converging 
rays refracted through a lens must yield the palm to parallel 
light which is obtained at the angle of total reflection in Newton’s 
plane prism as from the sun. 
Such, then, is the principle of illumination, and such the 
pencil of light appropriate to diatom work ; and in their appli- 
cation I have brought out that wondrous structure, which, 
without the aid of the microscope, must have remained among 
the invisible things of Him who created all things. 
MAGNIFYING POWER. 
In arranging my microscope for work, using a Ross’s ^th and 
a double D achromatic eye-piece, I pull out the eye-tube until 
the diameter of the field of view covers exactly the magnified 
image of xoVuth of an inch. This diameter measured on a foot- 
rule is exactly 12 inches at the distance of the stage from the 
eye. The magnifying power is, therefore, 12,000 linear, be- 
cause x^owth of an inch on the micrometer slide lying on the 
stage covers 12 inches on the foot-rule. These few particulars 
will remove any misapprehension as to the extent of my 
magnifying power. 
DESCRIPTION OF DIATOMS. 
I will now give a more detailed account of various species of 
the Naviculce , noticing particularly the great difference in the 
