1916] 



On Problems in Capillarity 



799 



The main features of the colour grading to he seen in such a film, 

 apart from the black stage which is obtained later, are two purple 

 to blue bands, more or less wide and separated according to the 

 amount of thinning which has occurred. A less distinct third band 

 is a redder shade of purple and is not so strikingly contrasted. This 

 banding is well adapted for showing the mobility of the film, as a 

 freely moving disc of liquid. When rotated in its own plane, the 

 thickest and heaviest part will of course remain at the bottom, while 

 the remainder of the film will be relatively placed according to its 

 decreasing mass, and the surface tension will retard any relative 

 motion of the parts. The colour bands, therefore, will keep horizontal 

 when the film bulb is rocked in the plane of the film. To show the 

 movement, some uniformly spaced black and white bands are marked 

 on a card surrounding the film and in the same plane (see Fig. 8). 

 The card is fixed to the film bulb and moves with it when rocked, 



FiC4. 8. 



while the bands are seen to remain practically stationary. AVith a 

 suitable scale replacing the banded card, the amount of displacement 

 from the horizontal can be read. The film can thus be used as a level, 

 and from its permanence and easy production finds a ready apphca- 

 tion to such a purpose. A half black film in which the lower half is 

 too thick to show colour is a better index than the varying colour 

 bands, and gives instead a bright contrast in a very mobile condition. 

 This is leadily obtained from the totally black film, usually found in 

 a bulb which has been left for a short time. The liquid for providing 

 the thicker uncoloured part comes from the excess present in the Gibb 

 ring. A quick rocking motion or oscillation in the plane of the film 

 soon causes enough of the excess liquid to be transferred to the film, 

 {ind instead of becoming uniformly distributed, it steadily accumulates 

 upwards over the black film to any desired amount. Finally, of 



