192 Sir William Thomson [Feb. 2, 



of a soap-film in a ring of metal. The light is reflected from 

 the film filling that ring, and focused on the screen. It will show, 

 as you see, colours analogous to those of Newton's rings. As you 

 see the image it is upside down. The liquid streams down (up in 

 the image), and thins away from the highest point of the film. First 

 we see that brilliant green colour. It will become thinner and 

 thinner there, and will pass through beautiful gradations of colour 

 till you see, as now, a deep red, then much lighter, till it becomes a 

 dusky, yellowish-white, then green, and blue, and deep violet, and 

 lastly black, but after you see the black spot it very soon bursts. 

 The film itself seems to begin to lose its tension, when it gets 

 considerably less than a quarter of the wave-lensjth of yellow light, 

 which is the thickness for the dusky white, preceding the final black. 

 "When you are washing your hands, you may make and deliberately 

 observe a film like this, in a ring formed by the forefingers and 

 thumbs of two hands, and watch the colours. Whenever you begin 

 to see a black spot or several black sj)ots, the film soon after breaks. 

 The film retains its strength until we come to the black spot, where 

 the thickness is clearly much less than 1-60, 000th of a centimetre, 

 which is the thickness of the dusky white.* 



Newton, in the following passage in his ' Optics' (pp. 187 and 191 

 of edition 1721, Second Book, Part I.), tells more of this important 

 phenomenon of the black spot than is known to many of the best of 

 modern observers. 



"Obs. 17. — If a bubble be blown with water, first made tenacious 

 by dissolving a little soap in it, it is a common observation that after 

 a while it will appear tinged with a variety of colours. To defend 

 these bubbles from being agitated by the external air (whereby their 

 colours are irregularly moved one among another so that no accurate 

 observation can be made of them), as soon as I had blown any of 

 them I covered it with a clear glass, and by that means its colours 

 emerged in a very regular order, like so many concentric rings 

 encompassing the top of the bubble. And as the bubble grew thinner 

 by the continual subsiding of the water, these rings dilated slowly 

 and overspread the whole bubble, descending in order to the bottom 

 of it, where they vanished successively. In the meanwhile, after all 



* Since this lecture was delivered a paper " On the Limiting Thickness of 

 Liquid Films," by Professors Reinold and Riicker, has been coniraunicated to 

 the Royal Society, and an abstract has been published in the 'Proceedings,' 

 No. 225, 1883. The authors give the following results for the thickness of a 

 black film of the liquids specitied : — 



Liquid. Method. Mean Thickness. 



Plateau's " Liquids Electrical. '119x10-^ cm. 



Glycerique." Optical. -107 „ 



Soap Solution. Electrical. -117 „ 



Optical. • 121 „ 

 The thickness, therefore, of a film of the liquide glyce'rique and that of a film 



of a soap solution containing no glycerine are nearly the same, and about 

 l-50th of the wave-length of sodium light. 



