388 A. A. C. ELIOT MERLIN ON THE MINIMUM VISIBLE. 



may enable him to detect Nos. 2 and 3 by daylight. Lamplight, 

 unless its yellow tint be subdued with a blue chimney and other 

 blue glasses, extinguishes the dot by the flame image produced by 

 the diatomic lens. It may be recovered, however, in front of it 

 by careful manipulation." ..." Dr. Van Heurck obligingly 

 photographed with the new apochromatic glass the eidolic dot 

 shown by the beading of P. angulatum." 



Dr. Royston-Pigott estimates the dots in P. angulatum to be 

 attenuated to 1 /250,000th of an inch and considers that 

 extremely minute dots, about 1/300, 000th, are not only found 

 amongst diatoms, but reveal themselves in the transparent 

 headings of moth-scales, and adds, " but there are many forms 

 of these dots." It is also remarked that " exquisitely small and 

 black dots can often be seen in focal planes elevated slightly 

 above diatomic beads by using a black central stop below the 

 condenser. It requires very grand glasses to display these elegant 

 results." It is needless to point out that the late Dr. Royston- 

 Pigott was an upholder of the now abandoned view that the 

 perforations of diatoms were solid silex beads or bosses. The 

 foregoing sufficiently proves that the central eidolic dots or 

 " pores " of diatoms were well known twenty-six years ago, but 

 those specially interested in the subject should read the papers 

 referred to. 



Setting aside all such diffraction phenomena, or false ghosts, 

 probably the most delicate, true diatomic structures just within 

 the grasp of our finest modern objectives of large aperture are 

 the thin perforated " veils "' to be detected on certain diatoms. 

 Of these perhaps one of the best examples is Triceratium america- 

 nitm, var., Oamaru, mounted in styrax by M oiler. It is a difficult 

 structure with axial screen illumination, but there can be little or 

 no doubt that the appearances observable represent real perfora- 

 tions in a thin outer plate. In this diatom there is no complicated 

 structure to bewilder the observer and manufacture false ghosts. 

 It is, however, extremely improbable that the minute perforations 

 of the IViceratium americanum, difficult as they are, represent 

 anything smaller than the 1/1 00,000th of an inch, and being 

 subject to the limitations of the laws of diffraction, like 

 all periodic structures, are consequently of little help as 

 an example of the minimum visible under more favourable 

 conditions. 



