USE OF THE IMAGINATION. 421 
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, wo 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 expansion 1 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 ex- 
perience 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 mech- 
anism 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 aug- 
ment 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 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. 
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 knowl- 
edge and skilled experiment reveal. If in all the multi- 
plied varieties of these phenomena, including those of the 
most remote and entangled description, this fundamental 
conception always bring us face to face with the truth; if 
no contradiction to our deductions from it be found in 
external nature, but on all sides agreement and verifica- 
tion; if, moreover, as in the case of Conical Refraction and 
in other cases, it actually forces upon our attention phe- 
nomena which no eye had previously seen, and which no 
