93 



Mr. W. R. Traviss exhibited and gave a description, illustrated 

 by aid of blackboard diagrams, of an expanding central stop for 

 dark-ground illumination. The device proved of considerable 

 interest to members present. 



The Hon. Secretary read a " Note on New Diatom Structure," 

 communicated by Mr. A. A. C. Eliot Merlin, F.R.M.S. 



Mr. A. E. Conrady did not think the magnification quoted of 

 the photographs excessive on the numerical aperture employed, 

 although, if taken much further, the image, to keen eyes at 

 least, would soon become " fluffy/' The ratio quoted — say one 

 thousand times the N.A. — corresponded pretty closely with the 

 employment by astronomers of a power of 50 to the inch of 

 telescope aperture ; and, although under very good conditions it 

 would occasionally be possible to use 75 or even 100 to the inch, 

 if one went beyond such a ratio, say to the equivalent of 2,000 

 times the N.A. of a microscope objective, we should not see 

 anything we could not see better with less magnification, although 

 we might be able to see something quite new, which, however, 

 would not belong to the object ! 



Mr. Morland said he did not often use high powers on diatoms. 

 His principal object was to show the form of a species. He 

 thought that, occasionally at least, the extremely fine markings 

 of these organisms must be obliterated by the heroic methods 

 adopted in cleaning. 



Mr. Earland asked why it should be expected that diatoms of 

 the same species — or book species — should always have to contain 

 the same number of markings to the millimetre. We did not 

 find elsewhere in nature that markings were absolutely constant 

 in number and character. It seemed that if a diatomist found 

 a specimen with a few more striatums to the micron than 

 usual, he promptly made it a new species. 



The President, in moving the adoption of a vote of thanks to 

 Mr. Merlin for his paper, said they all knew that that gentleman 

 had exceptionally keen eyesight. He suggested the exceptional 

 keenness might possibly be due to some physical peculiarity in 

 the structure of the eye, particularly so when regard was had to 

 the fact, as he understood, that Mr. Merlin always used a screen 

 in observing visually. The limit of visibility to the unassisted 

 eye was usually given as ~o * n - ') but ne thought that very few 

 indeed had eyesight equal to this. 



