ANOTHER WORLD DOWN HERE. 45 



retina. There does not appear to be in this or other insects a tightly 

 stretched membrane which, like the membrane of our ear-di'um, is 

 fitted to take up bodily air-waves and vibrate responsively to them. 

 But it is evidently adapted to receive and concentrate some kind of 

 vibration or motion or tremor. 



What kind of motion can this be ? What kind of perception does 

 this curious organ suj^ply ? To answer these questions we must travel 

 beyond the strict limits of scientific induction and enter the fairy-land 

 of scientific imagination. We may wander here in safety, provided 

 we always 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 ad- 

 jective is important, because there are vibratory movements of matter 

 that are not bodily but molecular. An analogy may help to render 

 this distinction intelligible. I may take a long string of beads and 

 shake it into wave-like movements, the waves being formed by the 

 movements of the whole string. We may now conceive 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 in- 

 visibly transmitted all along the line, and the outer ball will respond 

 by starting forward. 



Heat, light, and electricity, are mysterious internal movements of 

 what we call matter (some say " ether," which is but a name for im- 

 aginary matter). These internal movements are as invisible as those 

 of the intermediate billiard-balls ; but if there be a line of molecules 

 actino; thus, and the terminal one strikes an oro;an 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 j^roduced. When others of greater 

 amplitude and smaller frequency strike the terminal outspread of our 

 common sensory nerves, the sensation of heat results. The difference 

 between the frequency and amplitude of the heat-waves and the light- 

 waves is but small, or, strictly speaking, there is no actual line of sep- 

 aration lying between them ; they run directly into each other. When 

 a piece of metal is gradually heated, it is first " black-hot " ; this is 

 while the waves or molecular tremblings are of a certain amplitude 

 and frequency ; as the frequency increases, and amplitude diminishes 

 (or, to borrow from musical teiTQS, as the pitch rises), the metal be- 

 comes dull red-hot ; greater rapidity, cherry-red ; greater still, bright- 

 red ; then yellow-hot and white-hot : the luminosity growing as the 

 rapidity of molecular vibration increases. 



