ARTIFICIAL SKY 117 



into two ghost-like films, shaped like parasols. Thej 

 were barely visible, being of an exceedingly delicate blue 

 tint. They seemed woven of blue air. To compare them 

 with cobweb or with gauze would be to liken them to 

 something infinitely grosser than themselves. 



In all cases a distant candle-flame, when looked at 

 through the cloud, was sensibly undimmed. 



§ 2. On the Blue Colob op the Sky, and the 

 polaeization op skylight* 



1869 



After the communication to the Eoyal Society of tha 

 foregoing brief account of a new Series of Chemical Eeac- 

 tions produced by Light, the experiments upon this sub- 

 ject were 'continued, the number of substances thus acted 

 on being considerably increased. 



I now, however, beg to direct attention to two ques- 

 tions glanced at incidentally in the preceding pages — the 

 blue color of the sky, and the polarization of skylight. 

 Eeserving the historic treatment of the subject for a more 

 fitting occasion, I would merely mention now that these 

 questions constitute, in the opinion of our most eminent 

 authorities, the two great standing enigmas of meteor- 

 ology. Indeed it was the interest manifested in them by 

 Sir John Herschel, in a letter of singular speculative 

 power, addressed to myself, that caused me to enter upon 

 the consideration of these questions so soon. 



The apparatus with which I work consists, as already 

 stated, of a glass tube about a yard in length, and from 



* In my **Lectures on Light" (Longmans), the polarization of light will be 

 found briefly, but, I trust clearly explained. 



