ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 971 



and its accessories were so numerous, so beautiful, and so useful as to 

 excite the enthusiasm of all who used the instrument. He had made 

 himself an expert in the construction of object-glasses, and in every 

 department of his activity he had with a noble disinterestedness 

 made the world a free gift of his inventions." 



We must take exception to one remark of Mr. Cox. It is not 

 correct to say that Mr. Tolles " practically had to contend with the 

 organized authority of the Eoyal Microscopical Society." The 

 authority of the Society was never involved in the controversy, and 

 the Fellows who saw the absurdity of the denial of tbe existence of 

 an aperture in excess of that of 180^ angular in air were at all times 

 as numerous and influential, to say the least, as those who maintained 

 the contrary view. 



The only satisfactory point about this aperture question is that it 

 is at last at rest, and that it is now no more incumbent upon micro- 

 scopists to debate tlie question with objectors than it is for physicists 

 to debate the rotundity of the earth or its rotation upon its axis. 



Amphipleura pellucida resolved into "Beads." Nature of the 

 Striae of Diatoms. — Dr. H. Van Heurck writes to us as follows : — 



The A. pellucida has a double system of striation, transverse and 

 longitudinaljwhich has been known for some time, although the number 

 of observers who have seen the longitudinal striae is very limited. 



Hitherto tbe " beads " on this diatom have not been clearly re- 

 solved, and the possibility of exhibiting them has been doubted. The 

 matter is no longer doubtful, for I now adduce unmistakable proof — 

 a photograph of the " beads." 



In October 1883 I succeeded in producing a print on which tho 

 beads were fairly indicated, but the matter was not ripe for publica- 

 tion ; I was proceeding with further experiments, when I was attacked 

 by severe illness which prevented me from resuming my work during 

 the whole of the winter. 



I have recently taken up the subject again, and have succeeded 

 in obtaining photographs, both by transmitted light and by the 

 vertical illuminator, which suffice to clearly prove the existence of 

 the beads, although as photograplis they leave much to be desired. 



If these beads are difficult to observe distinctly, they are far more 

 difficult to photograph, so that I had almost despaired of obtaining 

 a satisfactory print. We m-.ij, it is true, succeed in viewing the 

 beads on the focusing screen of the photographic apparatus, and may 

 see them distinctly, and yet on developing the image on the sensitized 

 plate tlie wlude appears foggy, indistinct, and valueless. Out of 

 some fifty trials I hardly obtained one with tolerable success. I 

 used some of the best known objectives, such as the 1/12 and 1/18 

 of Zeiss, the 1/10 of Tolles, and the 1/8 (1-47 N.A.) of Powell and 

 Lcaland, all homogeneous immersions, and, notwithstanding, the 

 results were nearly always worthless. I attributed these failures to 

 the " chemical " focus of the objectives, but I have since found that 

 tho real explanation was in tho fact that tlio objectives were not equal 

 to the task. 



My success in photograpliing the beads has been due to the use 



3 s 2 



