150 
POPULAR SCIENCE REVIEW. 
usual rowlets, are very remarkable, and are found along the 
median and boundary lines, and in the open beading form 
clusters of dark irregular spots. The bead-shadows or crescents 
can be made to keep their original azimuth by revolving the 
stage ; proving their sphericity. I have only recently discovered 
that the median line of the formosum is composed of the four 
rows of minute beads shown in the figure. 
Fig. 2. — P. angulatum, nearly three times smaller than the 
formosum in its beading, presents similar characters in the 
main. 
Fig. 3. — The P. rhomboides can be beautifully shown with 
the immersion one-sixteenth, and probably equals the Nobert- 
line test in its exquisite dotted system, because a line is more 
easily seen than dots ; and a line of dots is more visible at a 
given distance in proportion as the dots are placed closer 
together. 
That the diatom spherules are veritable lenses of a convex 
form is evident from their appearance under the finest micro- 
scopes and from their optical properties, which justify the con- 
jecture that at no distant date they will be used to form focal 
images for the purpose of perfecting objectives themselves, when 
the greatest advance will have been made that opticians have 
ever accomplished in microscopical construction. 
