SCIENTIFIC USE OF THE IMAGINATION. 103 



time being. He breathes a space, and then renews 

 the struggle in another field. Now this period of 

 halting between two investigations is not always one of 

 pure repose. It is often a period of doubt and discom- 

 fort of gloom and ennui. ' The uncertainty where to 

 look for the next opening of discovery brings the pain 

 of conflict and the debility of indecision.' It was 

 under such conditions 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, 

 exercises 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 operations of Light and 

 Colour. 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 ? 

 Philosophers may be right in affirming that we cannot 

 transcend 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 purposes 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 employed. 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 



