312 J. DEBY ON THE STRUCTURE OF THE DIATOM VALVE. 



oblique light ; mounted dry or in media of refractive indices 

 varying from that of ordinary Canada balsam to the celebrated 2*4 

 of Prof. H. L. Smith. 



It is only by thus varying all the conditions of microscopic 

 vision and reasoning upon the various images produced, that any 

 hope can be entertained of forming a definite opinion, such as I 

 have formed, as to the real ultimate constitution of the Diatom 

 valve, one of the most difficult problems which the microscopist 

 can be called upon to solve. 



IV. 



The careful examination of good photographs has in many cases 

 been of great help to me in the interpretations of minute details. 



V. 



The presence of bubbles of air within the valves of a frustule, or 

 covered over by a single isolated valve standing on its free edge, 

 while fluids such as benzine are gradually poured upon it, and 

 which I have frequently noticed, preclude the existence of orifices 

 clear through the valve by which the gas would freely escape. I 

 have also never noticed bubbles of air within the areola of any 

 recent diatoms, while I have many slides of fossil diatoms with 

 corroded surfaces, where this phenomenon can be shown frequently 

 without any difficulty. 



VI. 



I have in my collection a series of well-mounted slides, which 

 have proved to my satisfaction the following facts most of which 

 are corroborative of previous observations by others : 



(a.) That the shell of most diatoms consists of a double plate. 



(b.) That between these two plates there exist a greater or 

 lesser number of cavities surrounded by solid walls of silica. These 

 cavities are circular or hexagonal in outline. 



(c.) That in all recent living and perfect valves the cavities are 

 closed at the top by the upper plate, and at the bottom by the 

 lower plate, and that these plates show no signs of orifices, but 

 only of thinnings over the cavities, except in abnormal cases where 

 the organic cuticle has been partially or totally destroyed by 

 accidental causes. 



(cZ.) That the external membrane is in most cases so slightly 

 silicious that even slight contact with acids promptly destroys it 

 and opens up the cavities at the back of it. That in other cases 



