151 



Note ox Double-Image Discs and Complementary Interference 



Colours. 



By J. Rheinberg, F.H.M.S. 



{Read April Idth, 1901.) 



Some years ago I was making experiments with multiple- 

 colour illumination, using discs above the objective ; and, in 

 making one or two such discs out of coloured glass, Messrs. Zeiss, 

 of Jena, observed a displacement of the image of the central 

 portion of the colour disc relatively to that of the peripheral 

 portion. They then made a double-image colour disc on purpose, 



in the manner indicated by the diagram, which shows the same 

 in section. 



A prism -shaped disc of green glass had a central hole perforated 

 in it, in which was inserted a smaller disc of red glass with plane 

 parallel surfaces. Consequently the light passing through the 

 prism-shaped part is shifted slightly as compared with that 

 passing through the centre, and we get" a separate image formed 

 by eaqh part. That formed by the central portion is an image 

 the same as if the aperture of the objective were cut down to the 

 same size. No fine detail would be seen with an objective of such 

 small aperture, and the image becomes a so-called "dioptric" one. 

 Provided the cone of light from the condenser does not exceed 

 this small aperture, nearly all the light which passes through the 

 prism-shaped portion of the disc will be light which is diffracted 



