The Microscope. 87 



Pleurodgma angulatuvi, in which I contended, against many distin- 

 guished diatomists, especially against my learned confrere and 

 and friend Dr A. Van Heurck, that the so-called pearls are not 

 hexagonal cells, deep like those of a honey-comb, but pearls as 

 indicated indeed by their name, that is, projecting granules. I 

 saw in those little arching grains elements of a sphere, that 

 should be hexagonal at the base though mutual pressure, but 

 round in their relief, that is, giving circular optical sections. I 

 reasoned not only on account of my personal observations, but 

 especially through exceedingly beautiful photographs, enlarge- 

 ments of plates obtained, some by Dr. H. Van Heurck with the 

 Zeiss apochromatic, N. A. 1.63, others by M. Ch. Basset with 

 Bezu and Hausser's water immersion rz inch objective. 



Among diatomists there are, on this subject, three camps ; the 

 first, now the most numerous, I believe, contending, with Dr H. 

 Van Heurck, for deep hexagonal depressions; the second, like 

 M. Ch. Basset and myself, arguing for projecting generally 

 rounded pearls ; the last belonging, as my friend G. Percheron 

 says, to the sect of the celebrated Chinese philosopher Ki San 

 Fou. 



Well, in the latest number of Le Diatomiste, issued by M. J. 

 Tempere, M. Leon Duchesne publishes an elaborate article on 

 the pearls of P. angulatum, basing his arguments upon the dif- 

 ferent results that he has obtained by micro-photography, while 

 working with the same objectives, under the same conditions 

 and over the same valve, but by slightly changing the point of 

 view. He thus proves that the pearls are in relief and round. 

 When the focal plane is tangent, or nearly so to the pearls in a 

 certain part of the valve, each bead is represented by a black 

 point (the summit that is in focus), surrounded by a white ring 

 (the rest of the pearl that is out of focus); as the focal plane is 

 lowered, each pearl gives a larger and larger circular black image 

 surrounded by a smaller and smaller white ring. It is a series 

 of comparative optical sections. 



As we continue to lower the focal plane, the relief of the pearl 

 ends by being no longer in focus, and we obtain an image of the 

 bead, or even a deeper image taken through the thickness of the 

 valve at the level at which the pearls originate. And this is 

 hexagonal, doubtless through reciprocal pressure. 



It is precisely this that I contended for and that he has de- 



