DEFINING AND PENETRATING POWER. 45 



scales thus obtained in Canada balsam, and illuminating them by means 

 of Wenham'a parabolic reflector. The structure may also be very 

 clearly recognised when the scale is seen as an opaque object under a 

 Eoss'a Jjth (specially adjusted for uncovered objects), illuminated by 

 a combination of the parabola and a flat Lieberkuhn. The under-side 

 of the scale thus appears as a smooth glistening surface, with very 

 slight markings, corresponding 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 Pleuro- 

 dgma; which, as arranged in the order of easy visibility, are, P. formo- 

 sum, P. hippocampus, P. angulatum. These appear to consist of a 

 lamina of homogeneous transparent silex, studded with rounded knobs 

 or protuberances, which, in P- formosum and P. angulatum, are ar- 

 ranged 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 de- 

 scribed ; and the visibility of these projections is probably propor- 

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

 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 consequently the substance is thinner there than elsewhere. 



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

 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. amgulatwm, and other similar objects, have 

 been by some writers supposed to be made up of two substances pos- 

 sessing different degrees of refractive power ; but this hypothesis is 

 purely gratuitous, since the observed phenomena will naturally result 

 from a series of rounded or lenticular protuberances of one homoge- 

 neous 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, it is difficult to conceive why the oblique 

 rays only should be visibly affected. When P. hippocampus or P. for- 



