MICROSCOPIC TEST OBJECTS. 
143 
relative diameters of the hemispheres, in their distance from 
oach other, and in their angular distribution. 
PLEUROSIGMA QUADRATUM. 
. Under my power of 12,000 linear Pleurosigma quadrature 
has 40 hemispheres and 40 intervals in the 12-inch diameter 
of the field, and, as each interval is equal to a radius of a hemi- 
sphere, the magnified diameter of each hemisphere covers T 2 Q-ths 
and the interval of an inch. Therefore, the real diameter 
of the hemispheres is - 6 0 * ^t h, and of each interval Q 0 th of 
an inch. The rows of hemispheres cross each other at an angle 
of 60°, as in P. angulatum , and are therefore arranged in the 
order of the sides of an equilateral triangle. Under common 
methods of illumination, these markings are described and 
figured as hexagons, and such indeed I have often seen them 
myself, but the illusion arises from causing either the illumi- 
nated or the shaded portions of the hemispheres to run into 
oach other, and so to form apparent hexagons with either dark 
or light centres. 
PLEURCSIGMA ANGULATUM. 
In Pleurosigma angulatum there are 48 hemispheres and 48 
intervals in the 12-inch diameter of the field. The diameter of 
the hemispheres is less than in quadratum , and the interval is 
less than a radius. On examination it appears that one hemi- 
sphere and interval cover £th, or -^-^-ths of an inch. The dia- 
meter of a hemisphere covers J^ths of an inch, and the width 
of an interval is y-g-^ths of an inch. Therefore the diameter is 
°f an inch (leaving only 1 after the fifth figure of 
the quotient for a remainder) instead of - 6 - 0 } p - pt h as in quad- 
ratum , and the width of an interval is T ( y * 0 o 5 th of an inch 
instead of as in quadratum. Larger hemispheres 
on each side of the strong median division add to the elegance 
of this favourite test, and, with the concentrated light of Brew- 
ster’s hemispherical prism, the beam being rendered parallel by 
means of a bull’s-eye lens, the light grey hemispheres of the 
valve, set off by the delicate pinJc-coloured silicious tissue, form 
a very charming object. 
NAVICULA RHOMBOIDES. 
In the Navicula rhomboides , a valve of exquisite beauty 
under prism illumination, there is an elegant border of three 
rows of beads or hemispheres gradually decreasing in size ; then, 
parallel to the median lice of the valve, 14 rows of much smaller 
beads, numbering at least 80 in T oVo^ °f an inch? and then 
