682 Prof. C. Y. Eaman and Mr. K. Seshagiri Rao on 



be strongly coloured*. Near the centre of the halo the 

 colours would not be very pure except in the case o£ 

 reiativelv thin films) owing to the increasing path-difference 

 nnder which the interferences take place, and especia ly 

 owing to the superposition of the two sets of rings in this 

 neighbourhood. These results agree with oh? erv.it ion. 



Fiff.l. 



Deviation. 



For a quantitative test of the correctness of the theory. 

 it is necessary to nse a monochromatic light source. A 



small aperture illuminated by a quartz mercury lamp, with 

 green raw filter is viewed from a distance through a mixed 

 plate of fairly uniform thickness, which may be obtained by 

 rubbing up egg-white between moderately thick flat plates 

 of glass and regulating the pressure over different pans of 

 the film in a suitable manner. The film is held normally 

 jlose to the eye., and moved about till one of the dark rings 

 in the halo just closes in on the centre. The part of the film 



* If the exact sequence of the colours in the outermost part of the 

 halo is to be inve^tigatedj the variation of refractive index p with 

 the ware-length X should, strictly speaking, also be taken into account 

 in making" the calculations. 



