508 SUMilARY OF CURRENT RESEARCHES RELATING TO 



perforations, as originally suggested by Mr. Stephenson. Were they 

 small elevations, they could not escape observation in exactly normal 

 sections, where, however, they were not seen. 



The true character of the markings may also be determined 

 without the sections. In preparations containing diatoms collected 

 by dissolving the rock, valves are found which are partly cleft, and 

 the upper layer of which is partly separated from the lower. The 

 former examined by itself shows only a network of hexagonal 

 meshes. The lower layer has a plane surface traversed by circular 

 openings, as is established by oblique light. If the preparation is 

 not dry, it is sufficient to press the cover-glass on the place where the 

 valve is in order to break it. The margins of the fractures present 

 the appearance shown in Fig. 115. 



As to the small points or circles figured on the surface of the 

 pretended elevations by certain authors, they are due to the efi"ect8 

 of diffraction, and disappear when an open areola is examined. The 

 same phenomena are observable in Trinacria jRegina, although less 

 distinctly, the openings of the latter being larger. 



Trinacria Begina Heib. was in the same way found to be 

 crowded with small circular apertures, as was evident in the fractures, 

 which had the same aspect as those of Coscinodiscus, and also from 

 longitudinal sections of canaliculi corresponding to the apertures. 



The nature of the sculpturing on the surface of the small diatom 

 Coscinodiscus excentricus was excessively difficult to determine clearly, 

 the examination of the fractures scarcely sufficing owing to the small- 

 ness of the areolae. 



The author points out that this method of examining diatoms is 

 also applicable to recent forms. By imbedding them in cement, 

 sections may be obtained by means of which many of the existing 

 difficulties will be solved, and the structure of these organisms better 

 understood. He has tried for P. angulatum a mixture of oxide of 

 zinc and silicate of potash, but although it polishes well it cannot be 

 cut sufficiently thin. 



Mr. J. Deby, on the other hand,* altogether contests the idea of 

 the perforation of the valves, and is convinced that the so-called 

 openings are an optical illusion. He bases this view mainly upon 

 observations of diatoms by reflected light, that is, as opaque objects.']' 

 An examination of such objects as Eupodiscus argus or Pleiirosigma for- 

 mosum, mounted dry without cover-glass, using a power of 1000 or 

 1500 diameters, and either the vertical illamiuator or the Woodward 

 prism, would, he thinks, for ever convince M. Prinz of the non-existence 

 of external orifices. In addition to his own observations, M. Deby refers 

 to the experiments first made in 1851 by Bailey, who dissolved diatoms 

 imder the Microscope in fluoric acid, and also to the collodion impres- 

 sions of Mr. Wenham. 



Another mode of showing that the openings are illusions is to 

 allow a highly coloured fluid to run in amongst some diatoms lying 

 dry on a slide. The fluid will be seen to be rapidly diffused (by 



* Kull. Soc. Belg. Micr., vii. (1881) pp. Ixxix.-lxxxii. 



t Sed qu. as to ojjaque illumination. See this Journal, ante, p. 364. 



