the Colours of Mixed Plates. 341 



out of step. A similar closing in of the rings is observed if 

 an electric arc with monochromator is used as the source and 

 the wave-length of the light incident on the plate is gradually 

 increased. 



Observations in White Light. 



In this case the halo has a fibrous radial structure, and 

 shows a series of coloured rings surrounding the source. The 

 outermost ring in the halo is very broad and practically 

 colourless or achromatic. The first few rings subsequent to 

 this achromatic ring within the halo are strongly coloured 

 even if a thick film be used, the succession of colours being 

 not very dissimilar to that observed in passing from the 

 achromatic centre to the coloured bands in the diffraction 

 pattern of theFrannhofer class due to a rectilinear slit. The 

 colours near the centre of the halo are very weak and impure, 

 unless the film is so thin that the total number of coloured 

 rings in the halo is rather small. In the latter case the halo 

 shows vivid colours, even near the centre. 



In all cases the source of light as seen through the mixed 

 plates appears enfeebled, as is evidently to be expected in 

 view of the fact that part of the incident energy appears 

 in the scattered light forming the halo. When the obser- 

 vations are mad'} in monochromatic light, and the plate is 

 moved in front of the eye so as to pass from a thick to a 

 thinner part of the film, the intensity of the source suffers 

 periodic fluctuations, being greatest when the innermost 

 dark ring in the halo is just about to close in at the centre 

 and least when one of the bright rings just surrounds the 

 source. When similar observations are made in white light. 

 the source appears to fluctuate both in intensity and colour 

 as the plate is moved so as to alter the thickness of the film 

 in front of the eye. The part of the halo immediately sur- 

 rounding the source and the source itself appear to be of 

 complementary colours. 



3. Haloes due to Obliquely-held Plates. 



If the plate instead of being held normally to the light is 

 gradually tilted, some very striking results are obtained, 

 especially when the observat ons are made in monochromatic 

 light. As the obliquity of the plate to the incident light is in- 

 creased, new rings appear at the centre of the halo and move 

 outwards : when a dark ring first appears it is very clear and 

 well-defined, but in moving out it becomes faint and broad. 

 and then again sharper and blacker, and so on. These 



Phil. Mag. S. 6. Vol. 41. No. 213. March 1921. 2 A 



