GOG T. F. SMITH ON THE VALVE OF PLEUROSIGMA. 



character completely changes. But more convincing than 

 all is, I think, a valve which displays a crack right across a 

 certain part; but is found to be sound when the focus is 

 shifted. 



My opinion is that the " hexagons," which have so long- 

 puzzled microscopists, points to the true structure of this 

 diatom, and that each valve is composed of two such layers, 

 the one super-imposed on the other. 



So far I recognize three types of structure in the Pleuro- 

 sigma. First, a valve composed of two layers of square grating, 

 such as Pleurosigma balticum ; secondly, a valve with two 

 layers of grating, with secondary markings placed diagonally, 

 such as Pleurosigma formosum ; and, thirdly, a valve with, two 

 layers of net-like structure, such as Pleurosigma angidatum, 

 and to one or the other of those types, I think, all the species 

 may be referred. 



In itself it may not be considered of great scientific import- 

 ance, whether a diatom possesses one, or more layers of struc- 

 ture, although even here, if a fact has to be known at all, it is 

 well to know it accurately ; but to me, its chief importance is 

 to be found in the lessons it teaches on the interpretation of 

 microscopic images generally. It teaches : First, that the want 

 of agreement of appearance with any particular theory, arose, 

 not from the impossibility of the objective giving a correct 

 image, but because the theory itself was wrong, and mistook a 

 part for the whole ; and, secondly, it teaches that the last word 

 has not yet been said on any structure, until it has been 

 thoroughly examined with an objective of the widest aperture. 

 The last word will not have been said even then, nor after the 

 use of any glass that will not show us the arrangement of 

 atoms which distinguishes living from non-living matter ; but 

 it will tell us this much, that although the structure shown 

 may be only the aggregation of parts altogether beyond the 

 ken of any objective, it will at least show those groupings in 

 the right place. 



EXPLANATION OF PLATE XXIII. 



Fig. 1. One of the outer membranes, consisting of a delicate grating 

 composed of alternate rings and squares. 

 „ 2. Outer membrane on other side of valve, consisting of a square 

 perforated grating. 



