754 THE POPULAR SCIENCE MONTHLY. 



index of the fluid. Some of the phenomena are, however, so curious 

 as to have led excellent observers like Brewster to reject the theory 

 of thin plates, and to assume the secretion of various kinds of coloring- 

 matter. If the rim of a wine-glass be dipped in soapy water, and then 

 held in a vertical position, horizontal bands soon begin to show at the 

 top of the film, and extend themselves gradually downward. Accord- 

 ing to Brewster, these bands are not formed by the " subsidence and 

 gradual thinning of the film," because they maintain their horizontal 

 position when the glass is turned round its axis. The experiment is 

 both easy and interesting ; but the conclusion drawn from it can not 

 be accepted. The fact is, that the various parts of the film can not 

 quickly alter their thickness, and hence when the glass is rotated they 

 rearrange themselves in order of superficial density, the thinner parts 

 floating up over or through the thicker parts. Only thus can the tend- 

 ency be satisfied for the center of gravity to assume the lowest pos- 

 sible position. 



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 Rein old and Riicker have recently made 

 the remarkable observation that the whole of the black region, soon 

 after its formation, is of uniform thickness, the passage from the black 

 to the colored portions being exceedingly abrupt. By two independ- 

 ent 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, al- 

 though in the main dependent upon principles long since established, 

 have imparted a new interest to the study of acoustics. The former, 

 apart from its uses in every-day life, has become in the hands of its 

 inventor, Graham Bell, and of Hughes, an instrument of first-class sci- 

 entific 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 changes 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 ex- 

 tending over a sensible area. If the surface of a solid remains undis- 

 turbed, 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 



