SCIENTIFIC USE OF THE IMAGINATION 



and to the possession of this property, 

 in so high a degree, the rapid trans- 

 mission of a sound-pulse through water 

 is to be ascribed. 



But water, as you know, is not neces- 

 sary to the conduction of sound ; air is 

 its most common vehicle. And you 

 know that when the air possesses the 

 particular density and elasticity corre- 

 sponding to the temperature of freezing 

 water, the velocity of sound in it is 

 1,090 feet a second. It is almost exactly 

 one-fourth of the velocity in water ; the 

 reason being that, though the greater 

 weight of the water tends to diminish 

 the velocity, the enormous molecular 

 elasticity of the liquid far more than 

 atones for the disadvantage due to weight. 

 By various contrivances we can compel 

 the vibrations of the air to declare them- 

 selves; we know the length and fre- 

 quency of the sonorous waves, and we 

 have also obtained great mastery over 

 the various methods by which the air is 

 thrown into vibration. We know the 

 phenomena and laws of vibrating rods, 

 of organ-pipes, strings, membranes, plates, 

 and bells. We can abolish one sound 

 by another. We know the physical 

 meaning of music and noise, of harmony 

 and discord. In short, as regards sound 

 in general, we have a very clear notion 

 of the external physical processes which 

 correspond to our sensations. 



In the phenomena of sound, we travel 

 a very little way from downright sensible 

 experience. Still the imagination is to 

 some extent exercised. The bodily eye, 

 for example, 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 extend our inquiries from the 

 auditory to the optic nerve. There is 

 in the human intellect a power of expan- 

 sion 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 purposes of light, an 

 adequately modified form of the mecha- 

 nism of sound. We know intimately 

 whereon the velocity 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 

 density constant we also augment the 

 velocity. A small density, therefore, and 

 a great elasticity, are the two things 

 necessary to rapid propagation. Now 

 light is known to move with the astound- 

 ing 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. Let us then carry our 

 results from the world of theory into the 

 world of sense, and see whether our 

 deductions do not issue in the very 

 phenomena of light which ordinary 

 knowledge and skilled experiment reveal. 

 If in all the multiplied varieties of these 

 phenomena, including those of the most 

 remote and entangled description, this 

 fundamental conception always brings 

 us face to face with the truth ; if no con- 

 tradiction to our deductions from it be 

 found in external nature, but on all sides 

 agreement and verification; if, more- 

 over, as in the case of Conical Refraction 

 and in other cases, it actually forces 

 upon our attention phenomena which 

 no eye had previously seen, and which 

 no mind had previously imagined such 

 a conception must, we think, be some- 

 thing more than a mere figment of the 

 scientific fancy. In forming it, that 

 composite and creative power, in which 

 reason and imagination are united, has, 

 we believe, led us into a world not less 



