44 SCIENCE IN SHORT CHAPTERS. 



not appear to be in this or other insects a tightly stretched 

 membrane which, like the membrane of our ear-drum, is 

 fitted to take up bodily air- waves and vibrate responsively 

 to them. But it is evidently adapted to receive and con- 

 centrate some kind of vibration, or motion, or tremor. 



What kind of motion can this be? What kind of per- 

 ception does this curious organ supply? To answer these 

 questions we must travel beyond the strict limits of scien- 

 tific induction and enter the fairyland of scientific imagi- 

 nation. We may wander here in safety, provided we al- 

 ways remember where we are, and keep a true course guided 

 by the compass-needle of demonstrable facts. 



I have said that the cornea-like membrane of the insect's 

 ear-bag does not appear capable of responding to bodily air- 

 waves. This adjective is important, because there are vi- 

 bratory movements of matter that are not bodily but 

 molecular. An analogy may help to render this distinc- 

 tion intelligible. I may take a long string of beads and 

 shake it into wavelike movements, the waves being formed 

 by the movements of the whole string. We may now con- 

 ceive another kind of movement or vibration by supposing 

 one bead to receive a blow pushing it forward, this push to 

 be communicated to the next, then to the third, and so 

 on, producing a minute running tremor passing from end 

 to end. This kind of action may be rendered visible by 

 laying a number of billiard balls or marbles in line and 

 bowling an outside ball against the end one of the row. 

 The impulse will be rapidly and invisibly transmitted all 

 along the line, and the outer ball will respond by starting 

 forward. 



Heat, light, and electricity are mysterious internal move- 

 ments of what we call matter (some say "ether," which is 

 but a name for imaginary matter). These internal move- 

 ments are as invisible as those of the intermediate billiard 

 balls ; but if there be a line of molecules acting thus, and 

 the terminal one strikes an organ of sense fitted to receive 

 its motion, some sort of perception may follow. When 

 such movements of certain frequency and amplitude strike 

 our organs of vision, the sensation of light is produced. 

 When others of greater amplitude and smaller frequency 



