864: Profs. C. V. Raman and Bhabonath Banerji on 



and finally appearing achromatic when viewed in a direction 

 nearly parallel to the plate. An edge too thick to show 

 colour when observed in a direction nearly normal to the 

 plate, will appear vividly coloured when viewed obliquely on 

 one side of this direction, and be practically invisible from 

 the other side. It should also be remarked that in oblique 

 directions the portions of any one curved boundary that can 

 be seen at a time become greatly reduced — in fact, nearly 

 contract to single points. The normal to the boundary at 

 these points lies in the plane of observation. Each element 

 of a curved boundary is therefore effective in scattering 

 light principally in a plane normal to its own direction *. 



For a closer examination of the manner in which the 

 colour of the light scattered by a laminar boundary varies 

 with the direction of observation, the mixed plate may be 

 placed on the table of a spectrometer, and viewed through a 

 low-power microscope which replaces the telescope ordinarily 

 used in the instrument. The laminar boundaries under 

 observation should be illuminated by a somewhat narrow 

 pencil normally incident on the plate, and in order to screen 

 this from entering directly into the field of view of the 

 microscope, a wire may be placed immediately in front of 

 the objective. On turning the microscope about the axis of 

 the spectrometer, the phenomena described in the preceding- 

 paragraph may be readily observed and studied. Viewed 

 nearly in the direction of the incident light, the laminar 

 boundaries appear of a uniform colour depending on the 

 thickness of the film. On turning the microscope aside to a 

 slightly oblique direction, each of the boundaries seen 

 changes colour, but differently in its two parts which are 

 seen respectively through the more and less refrangible 

 media. For instance, a closed boundary which, seen in a 

 nearly normal direction, appears throughout golden yellow, 

 viewed at a slightly greater obliquity, appears red on the 

 portion seen through the liquid and greenish blue on the 

 portion seen through air, the latter appearing much fainter. 

 The colours and intensities of the two parts of each boundary 

 are interchanged when the microscope is turned over to the 

 other side. Viewed at still greater obliquities, further 

 fluctuations of colour occur, the sequence of these variations 

 being quite asymmetric with respect to the two sides of the 

 direction of the regularly transmitted pencil. A clear idea 



* This is generally true of all curved diffracting boundaries on which 

 light is normally incident. See Phil. Mag. Jan. 1919, p. 127, and 

 Sept. 1919, p. 219. 



