LECTURES AND ESS A YS 



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 steam. 

 With accurate experiment and observa- 

 tion to work upon, Imagination becomes 

 the architect of phy si cal theory. Newton's 

 passage from a falling apple to a falling 

 moon was an act of the prepared imagina- 

 tion, without which the "laws of Kepler" 

 could never have been traced to their 

 foundations. Out of the facts of 

 chemistry the constructive imagination 

 of Dalton formed the atomic theory. 

 Davy was richly endowed with the 

 imaginative faculty, while with Faraday 

 its exercise was incessant, preceding, 



accompanying, and guiding all his experi- 

 ments. His strength and fertility as a 

 discoverer is to be referred in great part 

 to the stimulus of his imagination. 

 Scientific men fight shy of the word 

 because of its ultra-scientific connota- 

 tions ; but the fact is that without the 

 exercise of this power our knowledge of 

 nature would be a mere tabulation of 

 co-existences and sequences. We should 

 still believe in the succession of day and 

 night, of summer and winter; but the 

 conception of Force would vanish from 

 our universe ; causal relations would 

 disappear, and with them that science 

 which is now binding the parts of nature 

 to an organic whole. 



I should like to illustrate by a few 

 simple instances the. use that scientific 

 men have already made of this power of 

 imagination, and to indicate afterwards 

 some of the further uses that they are 

 likely to make of it. Let us begin with 

 the rudimentary experiences. Observe 

 the falling of heavy rain-drops into a 

 tranquil pond. Each drop as it strikes 

 the water becomes a centre of distur- 

 bance, from which a series of ring-ripples 

 expand outwards. Gravity and inertia 

 are the agents by which this wave-motion 

 is produced, and a rough experiment 

 will suffice to show that the rate of 

 propagation does not amount to a foot 

 a second. A series of slight mechanical 

 shocks is experienced by a body plunged 

 in the water, as the wavelets reach it in 

 succession. But a finer motion is at the 

 same time set up and propagated. If 

 the head and ears be immersed in the 

 water, as in an experiment of Franklin's, 

 the tick -of the drop is heard. Now, this 

 sonorous impulse is propagated, not at 

 the rate of a foot, but at the rate of 4,700 

 feet a second. In this case it is not the 

 gravity but the elasticity of the water 

 that comes into play. Every liquid 

 particle pushed against its neighbour 

 delivers up its motion with extreme 

 rapidity, and the pulse is propagated as 

 a thrill. The incompressibility of water, 

 as illustrated by the famous Florentine 

 experiment, is a measure of its elasticity; 



