16 J. RHEINBERC4 ON THE ORIGIN OF CERTAIN COLOUR 



mentioned, the thickness of a diatom is usually greater than any 

 films giving colours,* but the colours of the films are due to inter- 

 ference phenomena, whereby light of different wave-lengths 

 becomes successively extinguished ; and why should not the colours 

 of J. Ralfsii be due to some such cause? And this suggestion 

 gained in plausibility from the experiment of the coloured 

 dioptric beam ; for how could white light become coloured^ 

 except one of its component colours was extinguished, and how 

 could it be extinguished, when the object was colourless and 

 unpolarisable, except by interference? 



To an interference effect, therefore, the phenomena had to be 

 ascribed, and once that was settled, further matters began to 

 arrange themselves beautifully. Firstly, there w^as evidently 

 some connection w^ith the thickness and shape of the diatom, for 

 those which were apparently flat showed a more uniform tint 

 than the others. Then, again, a gradual merging of a valve from 

 purple to brown-yellow and yellow-green, corresponding with a 

 gradual thinning down of the valve, would be accounted for by a 

 successive extinction of green, indigo, and violet light. 



It now remained to be ascertained exactly in what manner 

 the interference arose, and that I think I can satisfactorily 

 demonstrate. 



First of all remember what diatoms, such as Actinocyclus Ralfsii y 

 essentially are from an optical point of view. They are thin 

 plates, maybe of uniform or of varying thickness, perforated 

 by a number of small holes (secondary or other structure we 

 will leave out of consideration, as not bearing on the points 

 to be established). Now, in regard to the diffraction efiects 

 presented by diatoms, what has been the position taken up ? 

 Diffraction effects are always worked out on the supposition 

 of an opaque body with openings, or of alternate opaque and 

 clear lines; and in treating of the diffraction phenomena pre- 

 sented by diatoms, it has invariably been tacitly assumed 

 that they behave like such an opaque body, as indeed in 

 many cases they happen to do, because no other effects arise 

 which complicate matters. But actually diatoms are not opaque : 



* A film of silex to show colours of thin plates under conditions perfectly 

 favourable for their production would have to be between about 5 and 40 

 millionths of an inch. I have ascertained by direct measurement that 

 Act mocychcs Ralfsii is nearer 200 millionths of an inch. 



