SCIENTIFIC USE OF THE IMAGINATION 



continuous throughout this discourse, 

 and of preventing either failure of know- 

 ledge or of memory from causing any 

 rent in our picture, I here propose to run 

 rapidly over a bit of ground which is 

 probably familiar to most of you, but 

 which I am anxious to make familiar to 

 you all. The waves generated in the 

 ether by the swinging atoms of luminous 

 bodies are of different lengths and ampli- 

 tudes. The amplitude is the width of 

 swing of the individual particles of the 

 waves. In water-waves it is the vertical 

 height of the crest above the trough, 

 while the length of the wave is the hori- 

 zontal distance between two consecutive 

 crests. The aggregate of waves emitted 

 by the sun may be broadly divided into 

 two classes : the one class competent, 

 the other incompetent, to excite vision. 

 But the light-producing waves differ 

 markedly among themselves in size, form, 

 and force. The length of the largest of 

 these waves is about twice that of the 

 smallest, but the amplitude of the largest 

 is probably a hundred times that of the 

 smallest. Now the force or energy of 

 the wave, which, expressed with reference 

 to sensation, means the intensity of the 

 light, is proportional to the square of the 

 amplitude. Hence the amplitude being 

 one-hundred-fold, the energy of the 

 largest light-giving waves would be ten- 

 thousand-fold that of the smallest. This 

 is not improbable. I use these figures 

 not with a view to numerical accuracy, 

 but to give you definite ideas of the dif- 

 ferences that probably exist among the 

 light-giving waves. And if we take the 

 whole range of solar radiation into 

 account its non-visual as well as its 

 visual waves I think it probable that 

 the force, or energy, of the largest wave 

 is more than a million times that of the 

 smallest. 



Turned into their equivalents of sensa- 

 tion, the different light-waves produce 

 different colours. Red, for example, is 

 produced by the largest waves, violet by 

 the smallest, while green is produced by 

 a wave of intermediate length and ampli- 

 tude. On entering from air into a more 



highly refracting substance, such as _ 

 or water, or the sulphide of carbon, all 

 the waves are retarded, but the smallest 

 ones most. This furnishes a means of 

 separating the different classes of waves 

 from each other; in other words, of 

 analysing the light. Sent through a re- 

 fracting prism, the waves of the sun are 

 turned aside in different degrees from 

 their direct course, the red least, the 

 violet most. They are virtually pulled 

 asunder, and they paint upon a white 

 screen placed to receive them " the solar 

 spectrum." Strictly speaking, the spec- 

 trum embraces an infinity of colours ; 

 but the limits of language, and of our 

 powers of distinction, cause it to be 

 divided into seven segments : red, orange, 

 yellow, green, blue, indigo, violet. These 

 are the seven primary or prismatic colours. 



Separately, or mixed in various pro- 

 portions, the solar waves yield all the 

 colours observed in nature and employed 

 in art. Collectively, they give us the 

 impression of whiteness. Pure unsifted 

 solar light is white ; and, if all the wave- 

 constituents of such light be reduced in 

 the same proportion, the light, though 

 diminished in intensity, will still be white. 

 The whiteness of snow with the sun 

 shining upon it is barely tolerable to the 

 eye. The same snow under an overcast 

 firmament is still white. Such a firma- 

 ment enfeebles the light by reflecting it 

 upwards : and when we stand above a 

 cloud-field on an Alpine summit, for 

 instance, or on the top of Snowdon 

 and see, in the proper direction, the 

 sun shining on the clouds below us, they 

 appear dazzlingly white. Ordinary clouds, 

 in fact, divide the solar light impinging 

 on them into two parts a reflected part 

 and a transmitted part in each of which 

 the proportions of wave-motion which 

 produce the impression of whiteness are 

 sensibly preserved. 



It will be understood that the con- 

 dition of whiteness would fail if all the 

 waves were diminished equally, or by the 

 same absolute quantity. They must 

 be reduced proportionately, instead of 

 equally. If by the act of reflection the 



