68 THE MICROSCOPE. 



diatomacene, differs entirely from that above described ; 

 it will suffice for my present purpose to notice the 

 valves of three species only of the genus Pleurosigma ; 

 these, arranged in the order of easy visibility, are, 

 P. formosum, P. hippocampus, P. angulatum. All 

 appear to consist of laminae of homogeneous trans- 

 parent silex, studded with dots, or rather protuberances, 

 which in P. formosum and P. angulatum have a trian- 

 gular arrangement, and in hippocampus a quadrangular. 

 The " dots " have been described as depressions ; but 

 if the frustule be broken the 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, for 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 king- 

 doms ; as, for example, the Isthmia, which possess 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 objects, 

 have been by some investigators supposed to be made 

 up of two substances possessing different degrees of 

 refractive power ; but this hypothesis is purely specula- 

 tive, 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 lens of the eye, it is difficult to conceive why 

 oblique rays only should be visibly affected. When P. 

 hippocampus or P. formosum is illuminated by a proper 

 condenser, with a central stop placed under the lenses, 

 and viewed by a quarter-inch object-glass of 70 aper- 

 ture, both being accurately adjusted, we may observe 

 in succession, as the object-glass approaches the object, 

 first a series of well-defined bright dots ; secondly, a 

 scries of dark dots replacing these ; and thirdly, the 



