40 THE MICROSCOPICAL NEWS. 
region, or a part of the edge of the valve. Sections of a thickness 
equal to ca, ce, ad, be, fig. 14 will illustrate this case. If we are 
looking at a section having a thickness ca, for example, with a low- 
angle penetrating objective, the ribs appear invariably closed at the 
upper part by a membrane, in a word, it appears to resemble a 
section of cylinders. This image is produced by the objective 
showing at one and the same time the section of the ribs, and the 
lines representing those parts of the valve much more deeply 
situated.* The result is an image difficult to interpret in any 
other way than by the presence of cylinders. (Fig. 15). 
Fig. 15 is the view given with Zeiss’ dry 7. Fig. 16 represents 
the same section under Tolles’ ;4, homog. immersion. Fig. 17 
does not alter whatever objective may.be employed. But if an 
objective without penetration is employed on a very thick section, 
the planes lie at such distances apart as to prevent the various 
images confounding each other, and so this cause of error is 
eliminated. (Fig. 16). 
We have considered up to now, sections of considerable thick- 
ness. In those which are thinner, the edges of the valve, and the 
solid parts which run on the raphe are ground away, and there 
remains only the section of the ribs themselves 6d, fig. 14. 
An image is obtained as shown in fig. 17 which leaves no doubt 
of the presence of parallel elevations separated by spaces a little 
wider than their thickness. 
I need not insist on the utility of sections for the study of 
diatoms, it is by this means alone that we may expect to have exact 
notions of their structure. All the methods of sectioning are not 
equally good. Sections prepared by embedding the frustule in 
gum often give rise to false interpretation, the centres are not 
entirely solid, and so many of the more delicate parts are ground 
away and disappear. Suppose for a moment that the raphe to be . 
only a very deep channel, but closed towards the interior (as other 
observers admit)t by a fine membrane, the least violence will suffice 
to rupture the section at this spot, and to give the image of a 
crevisse traversing the thickness of the valve right through. This 
effect cannot be produced by the process before described, because 
* The drawing of Walker Arnott, reproduced by M. Burgess (oc. cit fig. 27), 
represents a fragment of a valve having a thickness equal to de of fig. 14. 
Perhaps it comprises the total thickness of the semi-valve. 
+Schmidt. Botanische Zeitung, 1872, p. 741, et. seg. 
