MAX SCHULTZE, ON THE DIATOM-VALVE. 131 
that although, upon gradually focussing down upon the sur- 
face, clear points, though not altogether precisely defined, 
come first into view, and then dark ones, the two sets of 
points do not correspond with each other, but that the dark 
points make their appearance Jetween the bright ones which 
first came into view. When the clear points are ill defined, 
the dark ones, on the other hand, are very distinctly shown, 
quadrangular in P. dalticum and hexagonal in P. angulatum. 
These dark points, again, when the tube is still further 
lowered, in their turn become bright, a second time to become 
dark when the tube is still further depressed. Lastly, the 
lowering of the tube being continued, we again have a sort 
of confused or blended image of bright pomts. Thus, for 
instance, in P. angulatum, in which the succession of dark, 
bright, dark, is very manifest, an indistinct appearance of 
bright points precedes all. 
The explanation of these alternating images is not easy, 
according to Welcker’s idea, though everything seems to the 
author to signify that it takes place in the following manner : 
—The dark points brought into view upon the accurate focus- 
ing arrived at by the lowering of the tube manifestly repre- 
sent depressions. The indistinct bright points by which they 
are preceded do not coincide exactly with them in position, but 
may rather be said to be contiguous to them, and to represent, 
consequently, the dorders of the depressions. The dark points 
which, more particularly in P. angulatum, are seen arranged 
with beautiful regularity over the whole surface of the valve, 
present, on the further lowering of the tube, a bright appear- 
ance, whilst at the same time their borders become darker. 
At this point the bottom of the depression may be said to 
have been reached. So far all is clear. The borders of the 
pits form the system of ridges, which by oblique illumination 
appear as the lines. The intermediate pits, either quadran- 
gular or hexagonal, according to the number of the ridges, are 
seen as dark points so long as their bottom is not distinctly 
in focus. But the borders also of the depressions, when the 
tube is raised, may appear like illuminated points, because 
at the spots where two or three ridges decussate, or where 
one of them is abruptly bent, the reflection of the light gives 
the deceptive appearance of a projecting eminence or point. 
This latter appearance, however, as has been stated, is of an 
indistinct kind. It follows, therefore, that neither spherical, 
conical, nor pyramidal elevations are the cause of the punc- 
tated appearance on the surface of the above-named species 
of Pleurosigma, although the decussating sets of ridges may 
VOL. I1l.—NEW SER, K 
