104 mA&MWy OF SCIENCE. 
air and subjected to the action of a solar or an electric beam, 
are thus decomposed, the products of decomposition hang- 
ing as liquid or solid particles in the beam which generates 
them. And here I must appeal to the inner vision already 
spoken of. Kemembering the different sizes of the waves 
of light, it is not difficult to see that our minute particles 
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 dif- 
ferent dimensions, but all exceedingly minute, passing 
through air laden with extremely small particles. It is 
plain that such particles, though scattering portions of all 
the waves, will exert their most conspicuous action upon 
the smallest ones; and that the color-sensation answering 
to the smallest waves in other words, the color blue 
will be predominant in the scattered light. This harmon- 
izes 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 colors of the 
spectrum; blue is merely the predominant color. 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 nat- 
ural sky, the artificial one shows all the colors of the spec- 
trum, but blue in excess. Mixing very small quantities of 
vapor with air, and bringing the decomposingluminous beam 
into action, we produce particles too small to shed any sen- 
sible 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 action of 
the light, the azure becomes less deep: while later on a 
milkiness, such as we often observe in nature, takes the 
place of the purer blue. Finally the particles become 
large enough to reflect all the light-waves, and then the 
suspended "actinic cloud" diffuses white light. 
It must occur to the reader that even in the absence of 
definite clouds there are considerable variations in the hue 
of the firmament. Everybody knows, moreover, that as 
the sky bends toward the horizon, the purer blue is im- 
paired. To measure the intensity of the color De Saussure 
invented a cyanometer, and Humboldt has given us a 
