DIATOMACE.E 593 



to leave no doubt as to the details of its structure. If we closely 

 examine the photographic image of a portion of P. angulatum, pro- 

 duced under a magnification of 4,300 diameters, and shown in Plate 

 X, taken from a photograph by Dr. Zeiss, it will, in the majority of 

 cases, leave perhaps little doubt that the valves are covered by the 

 beads or apertures in a decussate arrangement. We have, in the 

 judgment of Count Castracane, to do here with ' beads ' and not with 

 ' cavities.' But. from the recent advances of our knowledge, this by no 

 means follows; they may with high probability be considered per- 

 forations in the silex of the frustule. This is, indeed, placed almost 

 in the form of a demonstration by the interesting fact that Mr. C. 

 Haughtoii Gill succeeded in filling up the 'dots' or 'pearls' of the 

 Naviculce and the secondary markings of the discoid and other forms, 

 so as to give evidence that the filling must be deposited in cavities. 

 It is done by soaking clean diatoms in a solution of subnitrate of 

 mercury until their markings are filled with it ; then they are 

 immersed in sulphide of ammonium ; a double decomposition takes 

 place, by which black insoluble sulphide of mercury is produced, and 

 left in the minute cavities in which it certainly appears to be formed. 

 By observing the lines of fracture, which always follow the interval 

 between two rows of ' beads,' there will be much suggestion given to 

 the observer on this subject. Count Castracane. referring to Plate 

 X, asked, 'Would it have been possible to have srcn these pearl-like 

 objects isolated, if, instead of beads, we had had apertures or depres- 

 sions 1 ' We can only reply that misinterpretation on such a subject 

 is so possible that it is only by employing all the aids to interpretation 

 which ingenuity can place within our reach, that we can ever be certain 

 as to our visual interpretation of these minute phenomena. On the 

 other hand, the areolated valves of Triceratiumfavus (fig. 442) present 

 a line of fracture which traverses indifferently the hexagonal areola? 

 and the lines in relief which connect them. 



Dr. Tan Heurck has been able to employ the new lens made by 

 Abbe, having a numerical aperture of 1'63, upon his special subject, 

 the Diatomacece. He concludes that diatom valves consist of t \\ o 

 membranes or thin films and of an intermediate layer, the latter 

 fx'iitij pierced with openings. The outer membrane is delicate, and 

 may be easily destroyed by acids, friction, and the several proces>e^ 

 of ' cleaning.' When the openings or apertures of this interior portioi i 

 are arranged in alternate rows they assume the hexagonal form ; 

 when in straight rows then the openings are square or oblong. 



It is, however, due to Mr. T. F. Smith, who worked at this 

 subject for years, to say that he long maintained this view, and 

 has presented skilful photo-micrographs in support of his contention, 

 [n Plate I, fig. 1, we have a photograph of his, showing the inside 

 f a valve of P. angulatum magnified 1,750 diameters, and ex- 

 hibiting the ' postage-stamp ' fracture ; while in fig. 2, in the same 

 plate, we have the outside of P. angulatum, showing a different 

 structure ; and Mr. Smith has abundant evidence of the existence 

 of what he has so long maintained. 



By using the new lens of the great aperture of 1'63, Dr. Van 

 Heurck "has produced some remarkable photo-micrographs, which 



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