MAX SCHULTZE, ON THE DIATOM-VALVE. 125 
explained by reference to experiments with unequally heated 
or cooled glass-globules, starch, &e. The subject is also well 
illustrated by an experiment with minute homogeneous glass- 
globules, coated with successive layers of collodion, and which, 
when immersed in Canada balsam, exhibit very distinctly, 
under a Nicol’s prism, the property of negative double re- 
fraction. 
Returning now to the further consideration of the struc- 
tural and sculptural conditions exhibited in the artificial 
siliceous pellicles, and to the comparison of the latter with 
the apparently similar markings on the diatomaceous scales, 
Professor Schultze goes on to remark that the surface of the 
bodies in question, when their formation has taken place 
gradually and uninterruptedly, almost always presents minute, 
more or less sharply acuminate elevations, disposed in regular 
or irregular order. 
The smaller they are the more regular usually is their 
arrangement, instances of which will be seen in figs. 7, 8, 9, 
which represent the appearance seen in these pellicles under 
a magnifying power of 350 diameters. Viewed in front, these 
preparations closely resemble the valve of Pleurosigma angu- 
latum, as it appears under a power of 800 diameters with one 
of Amici’s or Hartnack’s immersion-lenses. The most marked 
difference is in fig. 9, which represents a side view, at the 
borders of which the elevations are distinctly visible, an ap- 
pearance which does not exist in the diatom in question. But 
a still finer punctation, visible only nnder the highest powers, 
may be observed in the thinner’ artificial pellicles, so that a 
series of test-objects for any lenses hitherto constructed might 
be chosen from among them. 
This finer kind of relief-sculpture, however, affords but 
little insight into its nature. The only impression conveyed 
by it is that the entire pellicle, or, at any rate, its outer sur- 
face, is constituted of minute, closely contiguous spherules, 
some of which are acuminate. 
Far more instructive are the larger hemispherical or conical 
elevations, which may frequently be seen, disposed with great 
regularity, as in figs. 3, 5 ; sometimes intermixed with smaller 
ones, either regularly or irregularly (fig. 10). These elevations 
are either regularly or irregularly hexagonal at the base, or 
even circular, as infig. 10. If the specimens be immersed in 
water, with the summits of the elevations pointing upwards, 
these appear as bright points in a dark field when the micro- 
scope is focussed accurately upon them, the appearances pre- 
sented varying, however, it is almost needless to say, accord- 
ing to the focal distance at which the object is viewed, though 
