ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
103 
are moreover extremely sensitive to yellow, and as long as this kind of 
light predominates, the excellence of their delineation is unsurpassable. 
By their aid sharp negatives can be obtained with a mineral oil lamp 
and the ordinary low-power objectives, and this without a filter. With 
sunlight, or as soon as the light becomes impregnated with many blue 
rays, they fail. 
(5) Microscopical Optics and Manipulation. 
Amphipleura pellucida and Pleurosigma angulatum. — Dr. H. Van 
Heurck sent for exhibition at the December meeting some remarkable 
photomicrographs of Amphipleura pellucida and Pleurosigma angulatum 
taken with the new Zeiss’s 1/10 in. objective of 1*63 N.A. 
Monochromatic sunlight was used. Medium for the preparations 2 * 4. 
For those of Amphipleura moderately oblique light was used with magni- 
fying 230wers of 2000 and 3000 diameters. The Pleurosigma photo- 
graphs were taken with strictly axial light and a small aperture of the 
diaphragm, the magnification being 3000, 10,000, and 15,000. 
The photographs of Amphipleura show the valve completely resolved 
in beads (cf. supra, p. 91), while those of Pleurosigma show details not 
hitherto observed. 
Dr. Van Heurck considers that his “conclusions as to Pleurosigma 
are now complete and positive and may be summed uj) as follows: — 
(1) The alveoles of P. angulatum are hexagonal, at any rate in the 
place where the. two layers of the valve unite. 
(2) The intermediate beads are produced by bad focusing of the 
angles of the alveoles.” 
The following is a translation of a communication made by Dr. 
Van Heurck to the Belgian Society of Microscopy : — 
“ I have the pleasure to submit a new series of photographs of 
Pleurosigma angulatum obtained with the 2*5 mm. objective 1’63 N.A. 
On studying this diatom attentively, 1 observed a very singular 
appearance ; the alveoles or beads showed themselves in the form of 
very minute points, and were surrounded by a ring of six secondary 
beads when each alveole was viewed separately. If, however, the 
whole valve was viewed it w'as seen that the secondary beads were really 
intermediate between two principal adjacent layers of the valves. 
I thought at first that this appearance of structure was new, but 
later I saw that a similar appearance existed on the margin of the 
valve photographed by Dr. E. Zeiss (5000 diameters), and which is 
figured in his ‘ Atlas of Photomicrography.’ 
Photograph No. 1 reproduces the above appearance. No. 2 shows it 
under a power of 10,000 and with an exact focus. In No. 3 the focus 
was purposely altered so as to show the secondary beads better. 
How is this structure to be explained ? 
If the photograph No. 2 is attentively observed it will be seen that 
the alveoles are not round as has been generally believed in modern 
times, but that they present sensible angles.* 
An absolutely exact focus (photograph No. 4) shows that the opinion 
of the old microscopists was well founded and that the alveoles are really 
hexagonal. 
This hexagonal form being admitted, an easy explanation is obtained 
