114 FRAGMENTS OF SCIENCE 



In the phenomena of sound, we travel a very little way 

 from downright sensible experience. Still the imagina- 

 tion is to some extent exercised. The bodily eye, for ex- 

 ample, cannot see the condensations and rarefactions of 

 the waves of sound. We construct them in thought, and 

 we believe as firmly in their existence as in that of the 

 air itself. But now our experience is to be carried into a 

 new region, where a new use is to be made of it. Having 

 mastered the cause and mechanism of sound, we desire to 

 know the cause and mechanism of light. We wish to ex- 

 tend our inquiries from the auditory to the optic nerve. 

 There is in the human intellect a power of expansion — I 

 might almost call it a power of creation — which is brought 

 into play by the simple brooding upon facts. The legend 

 of the spirit brooding over chaos may have originated in 

 experience of this power. In the case now before us it has 

 manifested itself by transplanting into space, for the pur- 

 poses of light, an adequately modified form of the mechan- 

 ism of sound. We know intimately whereon the veloc- 

 ity of sound depends. When we lessen the density of 

 the aerial medium, and preserve its elasticity constant, we 

 augment the velocity. When we heighten the elasticity, 

 and keep the dcDsity constant, we also augment the veloc- 

 ity. A small density, therefore, and a great' elasticity, are 

 the two things necessary to rapid propagation. Now light 

 is known to move with the astounding velocity of 186,000 

 miles a second. How is such a velocity to be obtained? 

 By boldly diffusing in space a medium of the requisite 

 tenuity and elasticity. 



Let us make such a medium our starting-point, and, 

 endowing it with one or two other necessary qualities, let 

 us handle it in accordance with strict mechanical laws. 



