258 E. M. NELSON ON THE INTERPRETATION OF 



proper method being the examination of diatoms such as 

 Triceratitim, etc., in section. To this I reply that diatoms are 

 of two kinds, those possessing a single structure and those 

 which have two. The Pleurosigma, Navicula, Schizonema, etc., 

 belong to the first class. They are boxes formed by a very 

 delicate silicious perforated membrane. I look on the median 

 line as a girder to strengthen the delicate membrane. All 

 these kinds of diatoms you will notice are small. 



When we come to larger diatoms, such as Triceratium, 

 Isthmia, Coscinodiscus, Actinoptychus, etc., we find a two-fold 

 structure. I regard the delicate perforated membrane as the 

 structure which is of primary importance to the living organism 

 inside, and the main areolations as of secondary importance, as 

 being girders for the support and protection of that delicate 

 perforated membrane. 



With regard to the sections, they only confirm what had been 

 found out before with a half -inch objective and the stereo- 

 scopic binocular, viz., the nature of the girder work. I very 

 carefully examined those of the Triceratium exhibited by 

 Messrs. Powell and Lealand at the Royal Microscopical Society, 

 under one of their oil -^ N.A. 1*43. The perforations in the 

 delicate membrane were wholly invisible. These perforations 

 when viewed on the ordinary valve, not in section, can be seen 

 as markings, with a common half -inch objective (mind, I do 

 not say you can see them as perforations with a half-inch) 

 — in section they were invisible under one of the finest objec- 

 tives ever made. It is committing a grave error to seek for a 

 solution of the nature of the delicate membrane by examining 

 those diatoms which have a double structure, because in those 

 diatoms the perforations in the delicate membrane are very 

 minute. 



The very fact of a diatom having a very strong girder frame- 

 work points to the probability of the perforated membrane 

 being very delicate. It is a wiser plan to examine those 

 diatoms which have a bold single structure, such as the P. 

 forraosum and the Tryhlionella punctata. 



To return to the bar, a curious feature of this minute object 

 is that it is exceedingly sensitive to focus ; the smallest appre- 

 ciable alteration in focus and it is gone. Until I saw this 

 object I was ignorant of the extreme delicacy of focus of the 



