DIATOMACE^. 



427 



the dots elongate crossways, presenting the appearance of 

 small short hands. The Pleurosigma angulatum (fig. 223), 

 was first discovered in the Humber; the lines upon its 

 surface resemble the most elegant tracery, which are 

 resolvable into raised minute dots. The markings are 

 seen to be longitudinal, transverse, and oblique. 



In the vicinity of Hull many very interesting varieties 

 of Diatomacece have been found, the beauty of the varied 

 forms of which are such as to delight the microscopist ; 



:s>£ 



Fig. 223. 



1, PUvrosigma angulatum. 2, Portion of the same, magnified 1200 diameters. 

 3, Portion of P. formosum, magnified 5500 diameters. 



at the same time some of them are highly useful, as form- 

 ing that class of test objects which are best calculated above 

 all others for determining the excellence and powers of 

 object-glasses. It has been shown by Mr. Sollitt that 

 the markings on some of the shells are so fine as to range 

 between the 30,000th and 130,000th of an inch; the 

 Pleurosigma strigilis having the strongest markings, and 

 the Pleurosigma acus the finest. 



As to the value of the Diatomacece as test-objects, it is 

 generally admitted, and since Mr. J. D. Sollitt first pro- 

 posed the use of their shells for this purpose, we append 

 his measurements of the lines : — 



Amphipleura Pellucida, or Acus, 130,000 in the inch, cross lines. 



,, Sigmoidea, 70,000 in the inch. 



Navicula Rhomboides, 111,000 in the inch, cross lines. 



