504 Mr. E. S. Johonnott on the Thickness of 



Fig. 2. 



Fig. 3. 



I 



1 



a, 



a 



vertical fringes of white light, the appearance is as shown in 

 fig. 3. (a) represents the appearance when the frames are 

 below the surface of the solution ; (b) when the films are first 

 formed. The fringes, of course, do not show within the 

 image of the frames, and only the broad horizontal bands of 

 the coloured films are seen. As soon as the black film forms 

 at the tops of all the frames the vertical fringes are seen 

 displaced within the frames, and take successively the appear- 

 ances shown in (c), (d), and (e). 



