SCIENTIFIC USE OF THE IMAGINATION. 133 



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

 ism of sound. We know intimately whereon the velocity 

 of sound depends. When we lessen the density of a medium 

 and preserve its elasticity constant we augment the velocity. 

 When we heighten the elasticity and keep the density con- 

 stant we also augment the velocity. A small density, 

 therefore, and a great elasticity, are the two things neces- 

 sary 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 dif- 

 fusing in space a medium of the requisite tenuity and 

 elasticity. 



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

 ing it with one or two other necessary qualities ; let us 

 handle it in accordance with strict mechanical laws ; let us 

 give to every step of our deduction the surety of the syl- 

 logism ; let us carry it thus forth from the world of imagi- 

 nation into 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 re- 

 veal. If in all the multiplied varieties of these phenomena, 

 including 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 deductions 

 from it be found in external Nature, but on all sides agree- 

 ment and verification ; if, moreover, as in the case of Coni- 

 cal Refraction and in other cases, it has actually forced 

 upon our attention phenomena which no eye had previously 

 seen, and which no mind had previously imagined, such a 

 conception, which never disappoints us, but always lands 

 us on the solid shores of fact, must, we think, be something 

 more than a mere figment of the scientific fancy. In form- 

 ing it that composite and creative unity in which reason 

 and imagination are together blent, has, we believe, led us 

 into a world not less real than that of the senses, and of 



