MAX SCHULTZE, ON THE DIATOM-VALVE. 133 
that is, with one of from 500 to 800 diameters. It is especially 
by obligue light, under which only it is generally the case that 
the sets of ridges appear as continuous strie, that the illusion 
becomes perfect that we are beholding sets of lines running 
in a perfectly direct course; whilst observation with direct 
illumination, provided that the lenses have sufficient defining 
power, discloses the true state of things. By such a light, 
with Hartnack’s immersion-lens No. 10, Fig. 11, represent- 
ing a portion of the surface of the valve of P. angulatum, 
was drawn. 
The author then proceeds briefly to discuss the nature of 
the appearances presented in the more coarsely marked 
diatoms, such as Coscinodiscus, Eupodiscus, Biddulphia, and 
Isthmia, and shows that these appearances may be explained 
in the same way as those of Pleurosigma. In Isthmia, more 
particularly, he says, it may be readily shown that the quad- 
rangular areole are not elevations on the surface, but rather 
holes in the valve, which consequently is formed of a kind of 
lattice-work, like that of many of the Polycystina. In Coscino- 
dicus and Eupodiscus the existence of a similar network is not 
so easily made out, that is to say, it is difficult to determine 
whether the areole represent actual openings in the valve, or, as 
is more probable, are not merely very thin spots. In specimens 
of these diatoms mounted in Canada balsam the appearances, 
explained according to Welcker’s plan, are not reconcilable 
with the view of the areolz being depressions, but the reverse. 
This the author explains by stating that the Welckerian phe- 
nomenon is reversed in objects immersed in a medium of 
greater refractive power than themselves. In specimens im- 
mersed in water, or dry, the true appearances are at once dis- 
played. 
It is thus shown that the sculpturing, both in the coarsely 
and in the finely marked diatom-valves, although at first 
sight apparently allied to what is seen on the surface of 
the siliceous pellicles, is in reality due to wholly different 
conditions.* 
* Tt may be as well here to recall to microscopical observers that, on the 
occasion of the reading of Dr. Wallich’s paper on the diatom valve (*Q. J. 
Mice. Se.,’ VIT1, p. 129), Mr. Wenham, whose opinion on any matter of the 
kind is of the greatest weight, stated that with an object-glass of his own 
construction, with a focal distance of 3th inch and a large aperture, he had 
ascertained beyond doubt that in P. azgulatum and some others the valves 
are composed wholly of spherical particles of silex, possessing high refrac- 
tive properties. And he showed how all the various optical appearances in 
the valves of the Diatomacex might be reconciled with the supposition that 
their structure was universally the same. It would be interesting to know 
whether Mr. Wenham retains this opinion.—Eps., 
