380 E. M. NELSON ON ACTINOCYCLUS RALFSII. 



with a lens of '25 N.A. no colour will be perceived, while the 

 H. stelliger under similar conditions will be brightly coloured ; 

 if the aperture be increased to "5 or '6 the H. stelliger will be 

 resolved, while the colour of the H. suhtilis will be an intense 

 blue. Now the resolution of the H. suhtilis may be accomplished 

 with a dry lens of -95 N.A., used critically, but as this lens 

 reveals nothing of the extremely delicate structure we are con- 

 sidering on Actinocyclus^ it stands to reason that the colour, 

 observed in Actinocyclus with quite a low aperture and with 

 transmitted light, cannot possibly be caused by this delicate 

 structure. To repeat the argument : — 



HyALODISCUS SUBTILIS. 



This diatom when viewed upon a dark ground, with a lens 

 whose aperture is -55 N.A., is coloured ; the structure which gives 

 rise to this colour can be resolved by a dry lens of '95 N. A. 



A CTINOCYCL us Ra LFSII. 



This diatom when viewed by transmitted light, with a lens 

 whose aperture is -25 N.A., is coloured; this colour remains when 

 the coarse structure on the diatom is resolved ; a dry lens of 

 •95 N.A., however critically used, is quite unable to resolve the 

 fine veil on this diatom. If this fine veil were the diffractor 

 which caused the coloration of this diatom, it would require a 

 lens with an aperture of at least '55 N.A. to develop this colour. 



Finally, all diffraction colours vanish with transmitted light, 

 but the colour of A. Ralfsii, with the exception of that on 

 its narrow margin, is only visible with transmitted light. 



In this narrow margin the single process or nodule is situated ; 

 this I find has a very finely perforated cap, very similar to 

 those of the Aulisci which have been previously described. The 

 resolution of this detail is exceedingly troublesome, and perhaps 

 it is one of the most difficult images the microscope, as at present 

 constituted, is capable of dealing with. 



Comparison of a typical diffeaction coloured diatom with 

 AN Actinocyclus Balfsii. Objective -J ; -3 N.A. 



Hyalodiscv.s stelliger. Actinocyclus Ralfsii. 



Transmitted light . . colourless . . coloured. 

 Dark ground . . . coloured . . colourless. 



Coarse structure resolved . colourless . . coloured. 



Journ. Quekett Microscopical Club, Ser. 2, Vol. VII. No. 47, Novemher 1900. 



