188 Transactions of the American Institute. 



merely by pulling back with my finger-nail for an instant one of the 

 spirals. 



The air with which we are surrounded is capable of transmitting 

 such waves as these, and, when they strike upon our ears, we say we 

 hear a sound. I cannot at all, to-night, even indicate the experiments 

 and reasoning which, many years ago, forced physicists to' believe in 

 the existence of these sound-waves, which they had never seen and 

 never hoped to see. But, perhaps, we may do better than thus to 

 consume our time. Let us ask ourselves what ought to be the appear- 

 ance of one of these sound-waves, out in the free air, and we will 

 suppose, for simplicity, that it is produced by a single impulse. You 

 know that, when a stone is thrown into a quiet pool of water, the 

 wave which is generated is circular / it expands itself over more and 

 more surface, but always retains the form of a circle, and analogy 

 tells us that our sound-wave ought also to expand in a circle ; that this 

 circle should exist not only horizontally, but upward, downward, in 

 all directions, or that our wave ought to be bounded by a great many 

 such circles. Now, bodies which are thus bounded we call spheres ; 

 the sound-wave ought, then, to be round, like a globe, and we must 

 think of it as a kind of hollow bubble, which is swiftly growing 

 larger. So much, theory would indicate. 



Wonderful as it may seem, the recent advances of science have 

 actually placed in our hands the means of rendering visible these 

 waves of sound; and, although it would be quite out of place for me 

 on this occasion to enter into a detailed account of the method by 

 which this magical triumph was achieved, still, you may be interested 

 in a few hints with regard to the general line of thought followed by 

 Professor Toepler, to whom we owe these beautiful observations. 



A sound-wave consists, as we have seen, of a layer of air w T hich is 

 more, and of another which is less compressed than the surrounding 

 atmosphere ; in fact, of a layer of denser and rarer air. Now, when 

 light which is traveling through the atmosphere meets with a denser 

 or a rarer layer, it is usually turned a little out of its straight path ; 

 a very little, but enough sometimes to render the layer actually visible, 

 if proper optical means are employed. Let me give a rough experi- 

 mental illustration of this : All the lenses of the magic lantern have 

 been removed, and the screen is illuminated by rays of light which 

 e?nanate from the ignited lime ; that is, which come from one point. 

 The bottle in my hand contains a few drops of sulphuric ether t and 

 the upper part of it is filled with its vapor, which, as you know, looks 

 just like the air, that is, it is invisible, is in fact a kind of air itself, 



