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



in realgar, under a very perfect recent 1/1 2th apochromat of 

 N.A. 1*4, employed with a magnification of 4,200. Mr. Brown's 

 central "pores" could be readily distinguished at a certain high 

 focus on the outer layer of the valve. But in the " pores " so 

 revealed I immediately recognised my old friends Dr. Boyston- 

 Pigott's " dark eidolic dots of interference." In thus frankly 

 stating my conviction, I am sure that Mr. Brown, as a veteran 

 observer, would wish me to pursue no other course. We are all 

 liable to make mistakes in the interpretation of diatomic struc- 

 ture, and the only hope of progress lies in friendly criticism and 

 the exchange of views. Although I consider Mr. Brown's central 

 " pores " of Pleurosigma balticum, Navicula serians and P. angu- 

 lation! to be clearly false ghosts, it is by no means unlikely that 

 the outer layers of these diatoms may be perforated with fine 

 secondary structure, like the forms with coarser primaries. 

 Under the most critical conditions, with T4 N.A. and a magnifi- 

 cation of 4,200, something of the kind has been seen both in 

 P. balticum and N. serians. These appearances, however, are 

 far more elusive and difficult than the eidolic central clots, and 

 quite different in aspect and position. So far as my experience 

 goes, capped diatomic primaries are always pierced by at least 

 three or four secondaries when any such structure is observable. 

 It may nevertheless be safely asserted that if the primaries of 

 P. angulatum are thus capped and pierced, the secondaries must 

 be as much beyond the grasp of our best lenses as are the eidolic 

 dots of A . pellucida. 



In order to show how similar are the observational conditions 

 described by Dr. Royston-Pigott as necessary for the proper 

 demonstration of eidolic clots to those specified by Mr. Brown 

 concerning his central diatomic " pores," I must quote Dr. Royston- 

 Pigott's remarks on the subject at some length. In "Micro- 

 scopical Advances " * it is stated : " With regard to attenuated 

 circles, nothing are more abundant in diatomic and scale 

 markings. If a spherule be l/60,000th of an inch, the black 

 marginal ring is generally about one-fifth of this, or 1 /300,000th 

 thick, ornamented with a minute central black clot. The clot and 

 its fellows are amongst the most interesting and surprising sights 

 in minute microscopy. Few glasses will show them. That a 



* English Mechanic, vol. xlviii. p. 209. 



