170 PROCEEDINGS OF THE SOCIETY. 



able to scatter tlic light to a considerable extent, and hence became 

 visible. He thought that tliis discovery would give a new value to 

 the vertical illuminator. Dr. Van Heurck had obtained the resolution 

 both with Powell and Lealand's 1/8 of 1 • 47 N. A., and with Zeiss' 1/18 

 homogeneous immersion, which had much less aperture, so that the 

 extreme aperture was not absolutely essential except as giving slightly 

 better results. Dr. Van Heurck also stated that the mere classing of 

 A. pellucida and A. Lindlieimeri in the same genus hardly expressed 

 the closeness of relationship which he believed to exist between them. 

 He thought it extremely probable that A. Lindlieimeri was the 

 sporangial form (reproduced by generation) of A. pellucida. 



The President said it was a matter of some interest in these 

 discussions on the resolution of Amphipleiira to know what was the 

 size of the frustule. He had a very large collection of the diatom, 

 ranging through a great number of sizes, and he thought there were 

 some intermediate ones in which it might be possible to get the 

 dotting. So that it was simply a matter of variation whether a frustule 

 was called Amphipleura pellucida or one of the larger forms. The 

 same relation might also be said to exist between Navicula rhomboides 

 and FrustuUa saxonica, the one being only a different development of 

 the other. 



Mr. Beck said he noticed that the paper spoke of the " beads " 

 of Amphipleura pellucida, and it also referred to these beads as 

 depressions. He thought that in these matters there was nothing 

 like being accurate, and he was liimself at a loss to know how a 

 " bead " could be a depression. If they looked like the small 

 particles of a smashed drop of quicksilver, then he could understand 

 the resemblance to beads, but it seemed to him to be a contradiction 

 to talk of them as depressions. 



Mr. Crisp said that in the paper the word " beads " was placed in 

 inverted commas, showing that the author meant only that they 

 looked like what were generally called " beads " by microscopists. 

 He considered them in reality to be cavities. 



Mr. Dowdeswell said that Dr. Maddux had suggested that the 

 failure to get a good photograph might arise from the expansion of 

 the slide by heat after focusing. 



Mr. E. M. Nelson said that he thought it was impossible to infer 

 anything as to Amphipleura from what was seen in the case of 

 Triceratium, as the markings which were analogous to those of 

 Triceratium were at the bottom of the pits. In Dr. Flogel's section 

 of Triceratium, as sho^Ti by Mr. Powell at the Conversazione, the 

 deep markings were very clearly seen, but it was only by very careful 

 looking that they could see some little marks at the bottom of the 

 pits. He thought that a section of a diatom was absolutely useless, 

 because they could see more of the structure in the whole. Taking 

 the larger form first — T. punctata — it looked as if there were a number 

 of straight bars of silex with a sort of ridge or depression, and a 

 number of minute columns between, like a section of a colliery pit, or 

 not unlike a ladder. In the coarser ones this could be seen with 

 central light, but in Amphipleura it was not possible to see them in 



