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SUMMARY OF CURRENT RESEARCHES RELATING TO 
of the secondary beads, which are produced by the imperfect focusing 
of the angles of the network, that is, by the places where two lines run 
into one another. To verify this hypothesis I have studied with the 
same objective a great number of large diatoms where the structure 
allows of no doubt, and I have found in Goscinodiscus excentricus the 
confirmation of my assertion. 
The structure of this diatom is well known. With low-power 
objectives it is seen to present large hexagons. The valve is very 
convex, and by regulating the focus suitably we can obtain at the same 
time all the appearances from the real hexagons to the isolated point 
surrounded by six illusory intermediate beads. It is this which is 
shown in photograph No. 5. 
The last photograjdi. No. 6, shows that the valve of Pleurosigma 
is formed of two layers, and that the alveoles are hollowed out in the 
substance of the valve. The photograph shows a valve where the lower 
layer bears, on a part of the surface only, a fragment of the upper layer. 
The hexagonal form of the alveoles is seen round the median nodule. 
It may therefore be considered that the form of the alveoles is hexagonal 
at the point of union of the two layers, and that each’ alveole terminates 
above and below very gradually in the form of a dome. This is what 
my latest researches seem to demonstrate. I hope to be able, as soon as 
I have leisure, to send photographs in support of this view. 
List of photographs : — 
1. Pleurosigma angulatum W. Sm. x 3000. 
2. Ditto — exact focus on the intermediate illusory beads — about 
X 10,000. 
3. Ditto — out of focus, to show the intermediate beads better — about 
X 10,000. 
4. Ditto — focus on the hexagon — about x 15,000. 
5. Ditto — sliowing partially the two layers of the valve. 
6. Goscinodiscus excentricus'' 
Structure of Diatom Valves. — Dr. Van Heurck also sends us the 
following paper : — 
In my ‘ Synopsis of Diatoms ’ I showed that in the large Grypto- 
Baphides, for instance, Goscinodiscus, we can clearly distinguish three 
j)arts : an upper membrane, a lower membrane, and an intermediate layer, 
and these may be identified when isolated, either wholly or partially, 
in certain gatherings. 
We know from Prof. Abbe’s theory that the Microscope alone does 
not enable us to determine with certainty the structure of minute forms. 
But though technical means fail us, we can still make estimations by 
analogy as we do in most of the sciences. The study of large forms 
authorizes us to infer that the structure of the more delicate forms may 
be identical or at least very similar. 
It has frequently happened that the examination of favourable fractures 
lias enabled me to confirm these views, and they have also been confirmed 
by careful observers, such as Deby, Cox, and others. The portion of 
the intermediate layer jihotographed by Mr. T. F. Smith, and figured as 
No. 5 in his note on the Pleurosigma valve, is a case in point that may 
be considered quite conclusive. 
The more recent and powerful optical means placed at our disposal 
by the house of Zeiss, of Jena, allow us to go a stojj further in the study 
