130 Dr. G. C. Wallich on the Markings of the Diatomacea. 



the exterior surface. The " diamond pattern " often seen on 

 household ware of cut glass will at once serve to illustrate the 

 kind of structure in question. 



Lastly, whilst we have four series of lines in all the Diatoms 

 just alluded to, we have only three in such as exhibit true hexa- 

 gonal areolation; and we cannot, under any modification of 

 light, or adjustment, or magnifying power, convert these six- 

 sided spaces, so long as they remain clearly defined, into diagonal 

 or any other kind of lines. Whensoever appearances present 

 themselves under the microscope which are known not to repre- 

 sent the true structure or markings of an object, we may rely 

 upon it that either the instrument or its adjustments are at 

 fault. With perfect objectives and proper adjustments, these 

 spurious appearances cannot occur. 



The subjoined admirable woodcuts illustrate, in an exact 

 manner, the structure of the two typical forms, P. angulatum 

 and P. balticum ; and it will be observed that, even on the plane 

 surface of the paper, by altering the position of the figures with 



■ififk 



LFkfkFk. 





Stnicture of Pleitrosigma 

 angulatum. 



Structure of Pleurosigma 

 balticum. 



reference to the source of light, the appearance of elevations or 

 depressions may be imparted to the facets at pleasure. Under 

 proper management, this structure may be made apparent, with 

 equal clearness and definition, either by means of a ^ inch or 

 j2 inch aperture. 



