$62 Profs. 0. V. Raman and Bhabonnth Banerji on 



pieces of good plate-glass, pressed together with a drop or 

 two of water mixed with air between them, are placed in 

 front of the achromatic lens. The film thus enclosed 

 between the glasses forms the " mixed plate," which is 

 observed through a telescope placed with its objective just 

 behind the focus of the achromatic lens. The appearance 

 of the film, as seen with this arrangement, depends on the 

 form of the aperture or stop regulating the admission of 

 light from the focal plane of the lens into the object-glass of 

 the observing-telescope: — (A) With an aperture placed cen- 

 trally in the focal plane so as to admit the light coining to 

 the geometrical focus but cutting off the diffracted light, 

 the water-air boundaries in the mixed plate appear as 

 coloured lines in a bright white field. (B) When a central 

 stop is placed symmetrically so as to cut off the light coming 

 geometrically to a focus, and an annular aperture surround- 

 ing the stop admits only diffracted light into the observing 

 telescope, the whole field appears dark, except the water-air 

 boundaries, which are seen apparently doubled, shining out 

 as two brightly-coloured lines of light running parallel to 

 each other and separated by a tine perfectly dark line 

 coinciding with the exact outline of the boundary. The 

 colours seen depend only on the thickness of the film, and 

 are independent of the size or shape of the boundaries. 

 They are complementary to those seen in case A. A magni- 

 fied photograph of the film under these conditions, showing 

 the apparent doubling of the boundaries, was reproduced in 

 fig. 4 of the Plate accompanying the first part of the paper. 

 (C) When, instead of a symmetrical annular aperture, we 

 have only a small aperture placed eccentrically in the 

 focal plane admitting diffracted light into the observing 

 telescope, then the full outlines of the water-air boundaries 

 are not seen, but only two small portions of each closed 

 boundary, such that the normals to the boundary at the two 

 points visible are parallel to the radius vector joining the 

 focus with the aperture placed in the focal plane. With 

 such an excentrically-placed aperture the phenomenon of the 

 doubling of the boundaries noticed under (B) does not occur, 

 and we merely get a single luminous coloured line in a dark 

 field running along the portions of the boundary visible. 



The phenomena described above are closely analogous to 

 those exhibited by the strise in mica *. The observations 

 show clearlv that the laminar boundaries in a mixed plate 



* P. N. Ghosh, loc. cit. 



