1 68 POPULAR LECTURES AND ADDRESSES. 



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 spots, 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 i-6o,oooth of a centimetre, which 

 is the thickness of the dusky white. 1 



Newton, in the following passage in his ' Optics ' 

 (pp. 187 and 191 of edition 1721, Second Book, 



1 Since this lecture was delivered a paper "On the ^Limiting 

 Thickness of Liquid Films," by Professors Reinold and Riicker, 

 has been communicated to the Royal Society, and an abstract has 

 been published in the Proceedings, April 19, 1883. The authors 

 give the following results for the thickness of a black film of the 

 liquids specified : 



The thickness, therefore, of a film of the "liquide glycerique" 

 and that of a film of a soap solution containing no glycerine are 

 nearly the same, and about i/5oth of the wave-length of sodium 

 light. 



