152 J. RHEINBERG ON DOUBLE-IMAGE DISCS. 



by the object, consequently the image formed by this part is a 

 so-called diftraction image. I may mention that with a double- 

 image disc this diffraction image is spurious, inasmuch as all fine 

 periodic structure will be doubled owing to the absence of the 

 dioptric beam as a component. 



Now, it struck me that a double-image disc made in this manner, 

 but of clear, uncoloured glass throughout, ought to further test 

 and exemplify a point in the theory of interference colours — viz., 

 that when the dioptric image of a diatom showed colour- due to 

 interference, the diffraction image of the first order should show 

 the complementary colour. That was a point I tried to make 

 out in my paper on the colours of Actinocyclus Ralfsii last 

 December. (^See Journal Q. 31. C, Yol. VIII., p. 13). 



I have now had such a disc made, and the result, as you will 

 see for yourselves by examining the slide of Actinocyclus Ralfsii 

 under the microscope this evening, has fully borne out the 

 anticipation. With these diatoms, which show such varied and 

 vivid colouring, it is a very striking effect to see the two 

 images of each diatom lying next to one another in precisely 

 complementary tints. 



. ^ 



Joara. Qu-:kett Microscopical Club, Ser. 2, yol. VIIL, No, 49, November 1901. 



