ON LIGHT. 



295 



(78.) As the thickness of a soap-bubble cannot be sub- 

 jected to direct measurement, it is impracticable, in its 

 case, to verify what must be considered the fundamental 

 principle of this explanation viz., the regular increase, 

 in arithmetical progression, of the thicknesses at which the 

 several successive black or bright rings appear. But of 

 this we may satisfy ourselves, by adopting a different 

 mode of producing and viewing them. When a spectacle- 

 glass or any other convex glass lens of long focus is laid 



Fig. & 



down upon a plane glass before an open window (both 

 being scrupulously clean and well polished), and a slight 

 pressure applied, the same dark spot, surrounded by the 

 same series of coloured rings, is seen, their centre being 

 at the point of contact of the glasses, where of course 

 their distance is ;///. If the focal length of the lens ])e 



