SCIENTIFIC USE OF THE IMAGINATION 111 



ditions that I had to equip myself for the hour and the 

 ordeal that are now come. 



The disciplines of common life are, in great part, exer- 

 cises in the relations of space, or in the mental grouping 

 of bodies in space; and, by such exercises, the public mind 

 is, to some extent, prepared for the reception of physical 

 conceptions. Assuming this preparation on your part, the 

 wish gradually grew within me to trace, and to enable 

 you to trace, some of the more occult features and opera- 

 tions of Light and Color. I wished, if possible, to take 

 you beyond the boundary of mere observation, into a 

 region where things are intellectually discerned, and to 

 show you there the hidden mechanism of optical action. 



But how are those hidden things to be revealed ? Phi- 

 losophers may be right in affirming that we cannot tran- 

 scend experience: we can, at all events, carry it a long 

 way from its origin. We can magnify, diminish, qualify, 

 and combine experiences, so as to render them fit for pur- 

 poses entirely new. In explaining sensible phenomena, 

 we habitually form mental images of the ultra-sensible. 

 There are Tories even in science who regard Imagination 

 as a faculty to be feared and avoided rather than em- 

 ployed. They have observed its action in weak vessels, 

 and are unduly impressed by its disasters. But they 

 might with equal justice point to exploded boilers as an 

 argument against the use of steam. With accurate ex- 

 periment and observation to work upon, Imagination 

 becomes the architect of physical theory. Newton's 

 passage from a falling apple to a falling moon was an 

 act of the prepared imagination, without which the "laws 

 of Kepler" could never have been traced to their fonnda- 



