Coscinodiscus asteromjjhalus. By T. W. Butcher. 725 



Mr. Nelson, is not to be conceded. One cannot forget that in his 

 paper on " Diatom Structure " he has already said : — " These ter- 

 tiaries are exceedingly minute." " This is a difficult image, not 

 only on account of its minuteness, but because of its liability to be 

 merely an interference image " ; and " In the case now before us. 

 however, its very minuteness prevents this [fracture-margin test] 

 being applied, for it would be quite impossible to see at the frac- 

 tured edge such a structure, even though it were several degrees 

 coarser." Yet now he says, " A membrane closing the pipe has 

 been seen by me upwards of twenty years ago ; it is so diaphanous 

 that its presence was only perceived from an edge in a broken 

 specimen." And, behold ! it is dotted with perforations. It is 

 for Mr. Nelson to supply the arguments against the suspicion that 

 he has been recording yet another interference image. 



Whilst in London last May, I had several days' study of the 

 slides under almost exactly similar conditions to those set forth by 

 Mr. Nelson in the notes just quoted. 



The stand was the Powell and Lealand, No. 1, tube-length 

 250 mm., oil-immersion, sub-stage condenser, N.A. 1-40, objective 

 (Zeiss) 3 mm. apochromatic, N.A. 1*40, oculars (compensating) 

 Zeiss, screens (a) Gifford " F " line and (b) Wratten and Wain- 

 wright's (D -f- H) of the " M " series. 



The owner, the late Lord Edward Spencer Churchill, believed 

 that the outfit was the one used by the late Dr. Henri van Heurck 

 for his last work on the Diatomaceae. No further guarantee of the 

 working quality of the apparatus could be desired. That the 

 judgment of this great observer had not been at fault was proved 

 forthwith. The quality of the images presented to the eye was 

 beyond criticism. Employing critical lamp-illumination through- 

 out, the various resolutions alreadv described were obtained with 

 ease, and came out magnificently clear. The fractures were all 

 carefully worked through, but in not one single field was there the 

 slightest trace of the sought-for membrane. 



I deal at this moment with another point. Whilst my photo- 

 micrographs were being examined at the May Meeting of the 

 Society, one of the senior Fellows passed the criticism " that the 

 absence of the membrane in my specimens was probably due to the 

 valves having been over-cleaned, and that it had been washed 

 away in the process of preparation for mounting." 



On hearing this strikingly novel suggestion of an almost in- 

 credible dexterity of manipulation by Mr. Firth — admittedly one 

 of the finest of living mounters — I at once replied, " that it was 

 much more likely that Mr. Nelson's ancient slide, upon which the 

 observations had been based, might be an imperfectly cleaned one 

 by a dead and gone mounter, who had not the accurate methods of 

 a modern worker, but that I would at once communicate with 

 Mr. Firth on the matter." There was no necessity to argue the 



