RayleigKs Address to the British Association. 107 



When the thickness of a film falls below a small fraction of the 

 length of a wave of light, the color disappears and is replaced by 

 an intense blackness. Professors Remold and Kiicker have recently 

 made the remarkable observation that the whole of the black region 

 Boon after its formation is of uniform thickness, the passage 

 from the black to the colored portions being exceedingly abrupt. 

 By two independent methods they have determined the thickness 

 of the black film to lie between seven and fourteen millionths of a 

 millimetre ; so that the thinnest films correspond to about one- 

 seventieth of a wave-length of light. The importance of these 

 results in regard to molecular theory is too obvious to be insisted 

 upon. 



The beautiful inventions of the telephone and the phonograph, 

 although in the main dependent upon principles long since estab- 

 lished, have imparted a new interest to the study of acoustios. 

 The former, apart from its uses in every-day life, has become in 

 the hands of its inventor, Graham Bell, and of Hughes, an instru- 

 ment of first-class scientific importance. The theory of its action 

 is still in some respects obscure, as is shown by the comparative 

 failure of the many attempts to improve it. In connection with 

 some explanations that have been offered, we do well to remember 

 that molecular change in solid masses are inaudible in themselves, 

 and can only be manifested to our ears by the generation of a to- 

 and-fro motion of the external surface extending over a sensible 

 area. If the surface of a solid remains undisturbed, our ears can 

 tell us nothing of what goes on in the interior. 



In theoretical acoustics progress has been steadily maintained, 

 and many phenomena which were obscure twenty or thirty years 

 ago have since received adequate explanation. If some important 

 practical questions remain unsolved, one reason is that they have 

 not yet been definitely stated. Almost everything in connection 

 with the ordinary use of our senses presents peculiar difficulties to 

 scientific investigation. Some kinds of information with regard 

 to their surroundings are of such paramount importance to succes- 

 sive generations of living beings, that they have learned to inter- 

 pret indications which, from a physical point of view, are of the 

 slenderest character. Every day we are in the habit of recognis- 

 ing, without much difficulty, the quarter from which a sound 

 proceeds, but by what steps we attain that end has not yet been 



