140 
POPULA.il SCIENCE BEVIEW. 
eclairage for the purpose of illuminating objects with achro- 
matic light. A modification of this apparatus is now supplied 
with all the best microscopes, and is known as the achromatic 
condenser. 
TIIE DIATOM-YALYE. 
The preceding observations on the two essential parts of a 
microscope clearly lead to the conclusion that we are now in 
possession of an instrument which is in every respect well cal- 
culated to exhibit all we want to know about an object we can 
neither see nor touch. 
Among Nature’s invisibilia let us select the Diatom-valve. 
A keen eye may indeed see here a minute atom, but no appa- 
rent outline; the elegant S-like shape of the Pie urosigmata 
cannot be made out, and a single hemisphere on the surface of 
a valve is absolutely invisible. How, then, with the aid of the 
microscope is the Diatom-valve described by the host of ob- 
servers ? So far from there being any uniformity of statement, 
we may almost say, Quot homines , tot sententice. The “ Trans- 
actions of the Microscopical Society ” contain a curious record 
of the Protean aspects described by different microscopists, and 
it is amusing to read of the ingenious modes of playing with 
the illuminating rays, so that the eye, fortified by a little previous 
theory, may see at will, in one and the same valve, either 
elevations or depressions, triangular, quadrangular or hexagonal 
dots, with rhomboids, pyramids, or spheres. In a valuable 
paper by Dr. Wallich, “ On the Development and Structure of 
the Diatom-Valve,” communicated to the Microscopical Society 
in March 1860, it is stated that “ in Pleurosigma foi'mosum 
there exists good evidence to prove that the interlinear spaces 
are occupied by elevated rhomboidal papillae, which present 
facetted surfaces, whereas in P. balticum , instead of rhomboidal 
•elevations, we have four-sided flattened pyramids, presenting as 
in the former case four sets of lines, of which those bounding 
the spaces, and not crossing them, are the predominant ones.” 
Hence it is evident, that iu spite of the all but perfect condition 
of our magnifying apparatus and the marvellous variety of 
our illuminating apparatus, the true structure of the Diatom- 
valve is the vexed question of the day. 
In entering upon the subject myself, I took for granted that 
one statement, on which all were agreed, was true, and that on 
the valve of the P. angulatum there are sets of three lines in 
the direction of the sides of an equilateral triangle, and formed 
probably by elevated ridges. The shadows of these lines were 
generally obtained by the circle of light in the common “stop 
lens;” but in such condenser every portion of light, except 
that at right angles to the lines to be resolved, is injurious. I 
