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 law r s ; 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 



