DEFINING AND PENETRATING POWER. 59 



glistening surface, with very slight markings, correspond- 

 ing, probably, to the points of insertion of the plates on 

 the contrary side. The minuteness and close proximity 

 of the epithelial plates will readily account for their being 

 a good test of definition, while their prominence renders 

 them independent of the separating power due to large 

 angle of aperture. 



The structure of the second class of test-objects above 

 mentioned differs entirely from that above described; it 

 will suffice for the present purpose to notice the valves of 

 three species only of the genus Pleurosigma; which, as 

 arranged in the order of easy visibility, are, P. formosum, 

 P. hippocampus, P. angulatum. These appear to consist 

 of a lamina of homogeneous transparent silex, studded 

 with rounded knobs of protuberances, which, in P. formosum 

 and P. angulatum, are arranged like a tier of round shot 

 in a triangular pile, and in hippocampus like a similar tier 

 in a quadrangular pile, as has frequently been described ; 

 and the visibility of these projections is probably propro- 

 tional to their convexity. The " dots " have by some been 

 supposed to be depressions ; this, however, is clearly not 

 the case, as fracture is invariably observed to take place 

 between the rows of dots, and not through them, as would 

 naturally occur if the dots were depressions, and conse- 

 quently the substance is thinner there than elsewhere. 



This, in fact, is always observed to take place in the 

 siliceous loricse of some of the border tribes that occupy a 

 sort of neutral, and yet not undisputed, ground between 

 the confines of the animal and vegetable kingdoms; as, 

 for example, the Isthmia, which possesses a reticulated 

 structure, with depressions between the meshes, somewhat 

 analogous to that which would result from pasting together 

 bobbin-net and tissue-paper. The valves of P. angulatum, 

 and similar other objects, have been by some writers sup- 

 posed to be made up of two substances possessing different 

 degrees of refractive power; but this hypothesis is purely gra- 

 tuitous, since the observed phenomena will naturally result 

 from a series of rounded or lenticular protuberances of one 

 homogeneous substance. Moreover, if the centres of the 

 markings were centres of greatest density, if, in fact, the 

 structure were at all analogous to that of the crystalline 



