148 FRAGMENTS OF SCIENCB 



our leisure. Like the natural sky, the artificial one shows 

 all the colors of the spectrum, but blue in excess. Mixing 

 very small quantities of vapor with air, and bringing the 

 decomposing luminous beam into action, we produce parti- 

 cles 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 action of the light, the azure becomes 

 less deep; while later on a milkiness, such as we often ob- 

 serve 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" diifuses 

 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 

 impaired. To measure the intensity of the color De Saus- 

 sure invented a cyanometer, and Humboldt has given us 

 a mathematical formula to express the diminution of the 

 blue, in arcs drawn east and west from the zenith down- 

 ward. This diminution is a natural consequence of the 

 predominance of coarser particles in the lower regions of 

 the atmosphere. Were the particles which produce the 

 purer celestial vault all swept away, we should, unless 

 helped by what has been called ** cosmic dust,*' look into 

 the blackness of celestial space. And were the whole 

 atmosphere abolished along with its suspended matter, we 

 should have the *' blackness" spangled with steady stars; 

 for the twinkling of the stars is caused by our atmos- 



