972 SUMMARY OP CURRENT RESEARCHES RELATING TO 



of the incandescent electric lamp; still, I hope to improve upon 

 these results. Drummond's light in my hands was not satisfactory. 



I send for comparison a print of A. Lindheimeri Grun., a species 

 intimately allied to A.pellucida, differing only in being larger and 

 in having bolder striation. The details shown on A. Lindheimeri 

 will assist the interpretation of the print of A. pellucida. 



It will be observed that in both species the longitudinal lines 

 are not straight but wavy, which is due to the fact that the beads or 

 alveoli are not opposite each other, but alternate irregularly. This 

 is also observed in the photographs (vide photo. E, negative No. 789) 

 produced by Dr. Woodward, of the Ithomhoides Van Heurchia 

 Breb. This arrangement of the striation combined with the presence 

 of the rudiment of the median nodule, which on my prints is well 

 seen, confirms the opinion given by Mr. Kitton in a note on the text 

 of my Synopsis, which he has been good enough to read, that the 

 genus Amphijpleura presents no essential generic character which 

 would differentiate it from the genus Van Heurchia (the existence of 

 the keels not being demonstrable and their notification appearing to 

 him due to an error of observation), the species of the genus J.mp/w'- 

 pleura should therefore be comprised in the genus Van Heurchia. 



I take advantage of this opportunity to explain my opinion on the 

 nature of the striae of diatoms, stride which in many cases are only 

 seen by the help of oblique condensers. 



I cannot admit that these striee are illusory. 



The beads of the diatoms are really alveoli or cavities in the 

 thickness of the valves; between the cavities are thickened parts, 

 and it is these thickenings which appear as striae. These strias are 

 stronger or weaker according to the separation of the alveoli, and also 

 according as the siliceous bands between them are more or less thick. 



I have explained this point in detail, as well as my other views on 

 the structure of the valves, on pages 3 5-7 of the text of my Synopsis, 

 which were printed early this summer and of which a copy is 

 deposited with the Secretary of the Belgian Microscopical Society. 



By way of summary of this note, I state : (1) that the A. pellucida 

 as well as the A. Lindheimeri consist of alveoli arranged in series at 

 right angles ; the alveoli are arranged in regular transverse series and 

 in wavy longitudinal series. (2) Our present objectives sufl&ce to 

 elucidate the structure of the diatom valves, provided we employ 

 media of sufficiently high refraction, and suitable illumination. 

 (3) The striae exhibited by an improper illumination or by an 

 objective whose aperture is too low to resolve the alveoli or the 

 "beads" ot A. pellucida, are due to the thickened parts of the valve, 

 that is, to the parts situated between the beads and the alveoli. 



The photographs I send herewith are : 



(1) A. pellucida produced with Powell and Lealand's oil-imm. 

 1/8, illuminated by incandescent electric lamp,* and the vertical 



* The illumination was obtained by means of the Nelson-Mnyall-Van Heurck 

 lamp, — thus I name the Nelson-Mayall lamp, in which I have replaced the lamp- 

 wick by a Swan lamp of 6 volts. The great facilitj- of movement provided in 

 this lamp renders it of much service in tliese extremely delicate researches. 



