24 J. RHEINBERG ON THE " COLOURS " OF ACTINOCYCLVS RALFSII. 



difference between the refractive index of the diatom and the 

 substance in which it is embedded. The less the difference, the 

 narrower will be the ring, because the critical angle becomes 

 greater, and reflection at the side only occurs when the ray 

 reaches it at a greater angle than that. If the medium be 

 denser than the diatom, then the ring will be formed on the 

 inner side of the perforation, as in Fig. 9 ; and if the diatom be 

 denser than the medium it will be on the outer side, as in Fig. 10. 



In conclusion, let me remark that a recognition of the possi- 

 bility of interference effects, other than diffraction, occurring in 

 microscopic objects is not in itself new, for it is mentioned 

 in the well-known work on the Microscope by Naegeli and 

 Schwendener. 



Since this paper was written I find there is a specific name 

 for the colour phenomena produced in the manner described. 

 They are termed •' The Colours of Mixed Plates ^ According to 

 Preston ("Theory of Light," p. 205) Dr. Thomas Young first 

 discovered in 1802 that by interposing water or butter between 

 two glass plates, so that globules of two different media were 

 formed, coloured fringes or rings were produced by transmitted 

 light. Other experimenters have used soap and water, white of 

 egg^ whipped cream, etc., for the production of the effect. I am 

 not aware that the colours of mixed plates have ever been referred 

 to except in connection with experiments such as these, but their 

 production by the natural agency of diatoms like A. Ralfsii far 

 surpasses their production by artificial means in brilliancy and 

 regularity of effect — notably in the production of uniform colour 

 over a comparatively large surface. 



Joarn. Qutkett M icroscojaical Club, Ser. 2, Vol. VIII. , No. 48, April 1901. 



