138 FRAGMENTS OF SCIENCE. 



mentioned whose vapours, when mixed with air and sub- 

 jected to the action of a solar or an electric beam, are 

 thus decomposed, the products of decomposition hanging 

 as liquid or solid particles in the beam which generates 

 them. And here I must appeal to the inner vision already 

 spoken of. Remembering the different sizes of the waves 

 of light, it is not difficult to see that our minute par- 

 ticles are larger with respect to some waves than to 

 others. In the case of water, for example, a pebble will 

 intercept and reflect a larger fractional part of a ripple 

 than of a larger wave. We have now to imagine light- 

 undulations of different dimensions, but all exceedingly 

 minute, passing through air laden with extremely small 

 particles. It is plain that such particles, though scat- 

 tering portions of all the waves, will exert their most 

 conspicuous action upon the smallest ones ; and that 

 the colour-sensation answering to the smallest waves 

 in other words, the colour blue wiH be predominant 

 in the scattered light. This harmonises perfectly with 

 what we observe in the firmament. The sky is blue, 

 but the blue is not pure. On looking at the sky 

 through a spectroscope, we observe all the colours of the 

 spectrum ; blue is merely the predominant colour. 

 By means of our artificial skies we can take, as it were, 

 the firmament in our hands and examine it at our 

 leisure. Like the natural sky, the artificial one shows 

 all the colours of the spectrum, but blue in excess. 

 Mixing very small quantities of vapour with air, and 

 bringing the decomposing luminous beam into action, 

 we produce particles too small to shed any sensible 

 light, but which may, and doubtless do, exert an action 

 on the ultra-violet waves of the spectrum. We can 

 watch these particles, or rather the space they occupy, 

 till they grow to a size able to yield the firmamental 

 azure. As the particles grow larger under the continued 



