N. E. BROWN ON THE STRUCTURE OF DIATOMS. 333 



magnification and with oblique illumination, the appearance of 

 the closing plate is as shown at fig. 1G, represented for effect as 

 at black-dot focus, and drawn and shaded by the same method 

 as fig. 15. If, however, it is examined by dark-ground illumi- 

 nation, and especially if the illuminator be decentred so as to 

 reflect the light from the under surface of the cover-glass upon 

 the diatom, the closing plates appear to be much more raised 

 than as seen by oblique light and nearly hemispherical ; which, 

 however, is the correct appearance I am unable to say. Both 

 forms of illumination distinctly demonstrate that the outer and 

 inner closing plates have their central part raised above their 

 marginal attachment, or, in other words, each closing plate is 

 separated from its neighbours by a furrow. Floegel and Miiller, 

 however, both represent the inner plate as perfectly flat and 

 even, and continuous with that of the adjoining chambers, and in 

 their drawings (which I think must be somewhat diagrammatic) 

 of considerable relative thickness. Floegel represents the inner 

 plate as containing small cavities in its substance, closed on all 

 sides. Miiller, in the figure he published in 1898, represents the 

 plate as having small perforations through its substance, whilst 

 in that published in 1899 he represents the plate as having 

 small concave pits extending half-way through its substance on 

 the side facing the interior of the diatom. This latter view is, 

 I believe, much more correct than the other two interpretations, 

 for I find that at a magnification of 3,000 diameters, when the 

 light is oblique, or reflected upon it from the inner surface of the 

 cover-glass, so that the plate is of a dull greyish-white colour, 

 it is clearly seen to have pit-like cavities in it closed by a 

 membrane which is probably situated at the other surface of 

 the plate. These pits can be clearly demonstrated by gently 

 moving the mirror, when the shadow formed by the wall of the 

 pit is seen to move round upon the membrane at the bottom of 

 the pit. The appearance of the pits as seen with the light 

 reflected upon them from the under surface of the cover-glass at 

 a magnification of 3,000 diameters, but enlarged to somewhere 

 about 10,000 diameters, is as represented at fig. 17. This mem- 

 brane under this form of illumination is white, and is probably 

 very thin. When viewed with central light and accurately in 

 focus, it appears more transparent than the thicker plate- 

 substance, and the light shows through it more brightly. But 



