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The Origin of certain Colour Phenomena typically shown by 



ACTINOCYCLVS RaLFSII. 

 By J. E.HEINBERG, F.R.M.S. 



{Read December I2th, 1900.) 



At our meeting on June 15th last, Mr. Nelson gave us an 

 interesting account of some peculiar colour effects shown by the 

 diatom Actinocychos Ralfsii (see Journ. Q.M.C., Yol. 7, jd. 377). 

 Under my microscope this evening is a very fine slide showing 

 a group of these diatoms, and it will be seen that the valves 

 mostly show a beautiful blue or purple colour when viewed with 

 a 1-inch objective; but some valves are pea-green, while others 

 are golden-yellow, and in many the centre is of one colour writh 

 rings of other colours around it. Now the remarkable fact which 

 Mr. Nelson pointed out is that these colours are only seen by 

 transmitted liglit ; with dark ground illumination, he told us, the 

 diatom appears colourless, and this is exactly the reverse effect 

 to that of other diatoms, such as species of Pleurosigma, Hyalo- 

 discus stelliger, etc., which are colourless by transmitted light, but 

 show colour on a dark ground owing to well-known diffraction 

 phenomena. Further, Mr. Nelson told us that the colour was 

 entirely controlled by the aperture of the objective, which must 

 not be much greater than -45 N.A. ; for, viewed with any 

 objective of larger aperture — for example, with an ordinary 

 ^-inch — the colours vanish. To quote Mr. Nelson's words (I.e., 

 p. 379) : " The question, then, is : What is the cause of the colour 

 in Actmocyclus Ralfsii ? Obviously it cannot be a diffraction colour 

 arising from the ordinary primary structure forming the ' rays,' 

 which give the diatom its name, because, as we have seen above, 

 when this structure is resolved the colour is still visible, and no 

 colour arising from diffraction is visible when the diffractor itself 

 is resolved. It cannot be due to pigment, for if it were it would 

 remain visible when the aperture was increased beyond '45 N.A. 

 It cannot be caused by thin plates, because it would require 



