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of light. We wish to extend our inquiries from the au- 

 ditory nerve to the optic nerve. Now 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 a 

 knowledge 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 mechanism of sound. We know intimately whereon 

 the velocity of sound depends. When we lessen the 

 density of a medium and preserve its elasticity con- 

 stant, we augment the velocity. When we high ten the 

 elasticity and keep the density constant, we also aug- 

 ment 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 185,000 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, endowing it 

 with one or two other necessary qualities ; let us handle 

 it in accordance with strict mechanical laws ; give to 

 every step of your deduction the surety of the syllogism ; 

 carry it thus forth from the world of imagination to the 

 world of sense, and see whether the final outcrop of the 

 deduction be not the very phenomena of light which 

 ordinary knowledge and skilled experiment reveal. If 

 in all the multiplied varieties of these phenomena, in- 

 cluding those of the most remote and entangled descrip- 

 tion, this fundamental conception always brings us face 

 to face with the truth ; if no contradiction to our deduc- 



