AMONGST THE SEAWEEDS. 



33 



these is given. It is called Pleurosigma formosum 

 (Fig. 8). For this object I must go to my cabinet 

 and take out one of the slides there stored, and 

 which has been properly prepared by a professional 

 mounter. 



The silicious valves of Diatoms are beautifully 

 marked with ridges, called costse, connected with an 

 inner membrane ; besides which there are numerous 

 dots, more or less minute, along the outside mem- 

 brane. Generally the dots are arranged in regular 

 lines or series, radiating from the median line or 

 costa. . With a low power these rows of dots look 



FIG. 8. Pleurosigma formosum. 



like continuous lines or striae. Supposing that the 

 diatom has been properly cleaned and well mounted, 

 the striae can be resolved into dots by objectives of 

 the requisite quality and magnifying power. As 

 the fineness of the dots varies in different species 

 (it is the character of these markings that determines 

 the species), it must not, of course, be expected that 

 a lens which will show the dots on one will neces- 

 sarily do so on another. Certain well understood 

 species, therefore, are usually adopted by the 

 microscopist for testing lenses, and some of these 

 are commonly referred to as to a sort of standard by 

 which they describe the capabilities of an objective. 

 The costa is always visible, then the lines or striae 



