378 E. M. NELSON ON ACTINOCYCLUS RALFSIL 



those still finer blue, and some finer still will appear violet or 

 even incligo. 



ISTow if the lens be changed for one whose aperture is '4: N.A., 

 those that were ruddy will be colourless, and the structure that 

 gave rise to the colour will be resolved, those that were green 

 will be ruddy, and those that were blue will have become green, 

 and so on. If a lens of still greater aperture be employed, 

 those that were originally green will become colourless and 

 will be resolved, and the colours of the others will be lowered 

 a step in the gamut. This law, which holds good with other 

 diatoms, quite breaks down with an Actinocyclus Rcdfsii, for if 

 we examine one on a dark ground with a low power those 

 parts which were brilliantly coloured blue Avith transmitted 

 light now become a golden yellow. Again, all other diatoms 

 lose their colour when the structure which gives rise to it by 

 diffraction is resolved, but with the A. Ralfsii the colour remains 

 although the structure is resolved; and lastly other diatoms 

 when viewed by axial transmitted light appear white, while 

 this is brilliantly coloured, provided that a lens of suitable 

 aperture be employed to examine it. The colour in this diatom 

 is visible with transmitted light provided that the aperture of the 

 objective used does not greatly exceed "45 N.A. ; the power of the 

 objective or eyepiece is of no consequence, the aperture of the lens 

 is the sole determining factor in the case, as may be proved by 

 manipulating an iris diaphragm at the back of the objective. 



There is a slide in my cabinet which contains both an Actino- 

 cyclus Ralfsii and a Hyalodiscus stelliger. This last diatom 

 has an ordinary sieve-like structure of about 35,000 per inch. 

 Now, these two diatoms act in precisely contrary manners, for 

 on a light field with ordinary transmitted light the Actinocyclus 

 is brilliantly coloured while the Hyalodiscus is colourless; but 

 on a dark ground the Hyalodiscus is coloured, and the Actino- 

 cyclus colourless. In short, the Hyalodiscus follows the rule 

 of all other diatoms, e.g., the Pleurosigrace, Naviculce, etc., and 

 behaves precisely like them. In Actinocyclus Ralfsii the only 

 part which follows this general diatomic rule is the narrow 

 margin w-hich, with transmitted light, is a golden yellow,* but on 

 a dark ground exhibits a blue-green tint; this is a diffraction 



* This colour may be somewhat erroneously described, as its golden 

 tint may be caused by the contrast with the brilliant blue close to it. 



