"682 Transactions of the Society. 



on all sides was the cause of the marking during the development of 

 the valve. Considering the criticisms Weiss's work has experienced 

 through the erroneous theory of furrows, sculptures, &c., the credit 

 due to him must be kept prominently in view. 



Of similar tendency are the works of Count Castracane, for 

 which I refer to Just's Botan. Jahresb., 1873. It will be equally 

 unnecessary to enter upon their contents. 



I have at various times in the course of this paper referred to 

 Prof. Pfitzer's epoch-making work (19). I am obliged to speak 

 of it once more, because at the conclusion, speaking generally of 

 the cell-envelope of diatoms (p. 174), he reproaches me with having 

 in mj Pleurosigma researches insufficiently estimated the possibility 

 that the connections between the two surfaces of the cell- wall are 

 distinguished from the interspaces by the stronger refractive power 

 due to the molecular constitution. Then he refers to the bast-cells 

 which have similar sculpture, and as worthy of notice he mentions 

 that with diatoms differences could not usually be due to water, 

 but to siHca-contents. I do not know Pfitzer's reason for this 

 statement ; I have fully explained (6, p. 474) that I have examined ' 

 fresh specimens which had been boiled in nitric acid and in chlorate 

 of potash, and further (p. 485) that through continued boiling the 

 sculpture does not alter. In the latter case, surely nothing else 

 but silicic acid remains ; then what does he mean by making a 

 difference by saying sihca-contents ? The transverse section of a 

 boiled valve shows exactly the same walls as previously. It 

 appears to me that my demonstration of closed cavities came to 

 Pfitzer's notice at an inopportune moment, because in the same 

 journal he brought forward his furrow theory which, as we have 

 seen above, is wrong in every respect. Since Pinnularia seems to 

 be the only diatom examined by Pfitzer by the section method, in 

 order to discover its sculpture, he adopted it as the type of diatom 

 sculpture, and when, soon afterwards, Muller proved real outer 

 openings in Triceratium and tried to make useless corrections of 

 my work, Pfitzer evidently believed that I had fallen into error, 

 and that his so-called surface-sculpture was a property common to 

 all diatom frustules, Pleurosigma included. In proof of this, one 

 need only glance at Pfitzer's subsequent writings. Let me only 

 draw attention to his latest (20), from which we might infer that 

 it expounded to some extent the latest views on the subject. But 

 about the structure of the cell- wall it contains nothing other 

 than Pfitzer's furrow theory, and 0. Mtiller's sculpture of 

 Triceratium. 



Another paper which we have to discuss is by Prof. Abbe: 

 ' Beitrage zur Theorie des Mikroskops.' In this work (1, p. 450) it 

 is demonstrated " that all the finer sculpture of an object, of which 

 the elements are small and close enough to produce by their 



