SCIENTIFIC USE OF THE IMAGINATION 



shivered off. The great mass of the 

 wave passes over such a particle without 

 reflection. Scatter, then, a handful of 

 such minute foreign particles in our 

 atmosphere, and set imagination to watch 

 their action upon the solar waves. Waves 

 of all sizes impinge upon the particles, 

 and you see at every collision a portion 

 of the impinging wave struck off; all the 

 waves of the spectrum, from the extreme 

 red to the extreme violet, being thus 

 acted upon. 



Remembering that the red waves stand 

 to the blue much in the relation of 

 billows to ripples, we have to consider 

 whether those extremely small particles 

 are competent to scatter all the waves in 

 the same proportion. If they be not 

 and a little reflection will make it clear 

 that they are not the production of 

 colour must be an incident of the scatter- 

 ing. Largeness is a thing of relation ; 

 and the smaller the wave, the greater is 

 the relative size of any particle on which 

 the wave impinges, and the greater also 

 the ratio of the portion scattered to the 

 total wave. A pebble, placed in the 

 way of the ring-ripples produced by 

 heavy rain-drops on a tranquil pond, will 

 scatter a large fraction of each ripple, 

 while the fractional part of a larger wave 

 thrown back by the same pebble might 

 be infinitesimal. Now we have already 

 made it clear to our minds that, to 

 preserve the solar light white, its con- 

 stituent proportions must not be altered ; 

 but in the act of division performed by 

 these very small particles the proportions 

 are altered ; an undue fraction of the 

 smaller waves is scattered by the particles, 

 and, as a consequence, in the scattered 

 light blue will be the predominant 

 colour. The other colours of the 

 spectrum must, to some extent, be 

 associated with the blue. They are not 

 absent, but deficient. We ought, in 

 fact, to have them all, but in diminishing 

 proportions, from the violet to the red. 



We have here presented a case to the 

 imagination, and, assuming the undu- 

 latory theory to be a reality, we have, I 

 think, fairly reasoned our way to the 



conclusion, that were particles, small in 

 comparison to the sizes of the ether 

 waves, sown in our atmosphere, the light 

 scattered by those particles would be 

 exactly such as we observe in our azure 

 skies. When this light is analysed, all 

 the colours of the spectrum are found, 

 and they are found in the proportions 

 indicated by our conclusion. Blue is 

 not the sole, but it is the predominant 

 colour. 



Let us now turn our attention to the 

 light which passes unscattered among 

 the particles. How must it be finally 

 affected ? By its successive collisions 

 with the particles the white light is more 

 and more robbed of its shorter waves ; 

 it therefore loses more and more of its 

 due proportion of blue. The result may 

 be anticipated. The transmitted light, 

 where short distances are involved, will 

 appear yellowish. But as the sun sinks 

 towards the horizon the atmospheric 

 distances increase, and consequently the- 

 number of the scattering particles. Thejr 

 abstract in succession the violet, the 

 indigo, the blue, and even disturb the pro- 

 portions of green. The transmitted light 

 under such circumstances must pass from 

 yellow through orange to red. This 

 also is exactly what we find in nature. 

 Thus, while the reflected light gives us 

 at noon the deep azure of the Alpine 

 skies, the transmitted light gives us at 

 sunset the warm crimson of the Alpine 

 snows. The phenomena certainly occur 

 as if our atmosphere were a medium 

 rendered slightly turbid by the mecha- 

 nical suspension of exceedingly small 

 foreign particles. 



Here, as before, we encounter our 

 sceptical "as if" It is one of the 

 parasites of science, ever at hand, and 

 ready to plant itself and sprout, if it can, 

 on the weak points of our philosophy. 

 But a strong constitution defies the 

 parasite, and in our case, as we question 

 the phenomena, probability grows like 

 growing health, until in the end the 

 malady of doubt is completely extirpated. 

 The first question that naturally arises is 

 this : Can small particles be really proved 



