THE DEVELOPMENT OF ILLUMINATION.' 



Bv Walter Hough. 



Before the period of artificial iliiiniination there were many mani- 

 festation.s of litjht in nature coming to the aid of the denizens of the 

 earth during the hours of darkness. Of these were the so-called luci- 

 form appearances, including the aurora l)orealis and australis, which 

 enliven the long nights at the polar zones; the magellan clouds of the 

 Southern Hemisphere; the zodiacal light, whose cause was long a sub- 

 ject of speculation, and the diffused light of the milky way, known to 

 the Chinese as the "river of the sky," 



The light from the stars and planets is not inconsiderable. Under 

 the clear night sky of the Arizona deserts the atmosphere seems 

 charged with star mist; eminences miles away may be outlined, the 

 dial of a watch ma}' l)e read,' and a trail followed with little difficulty. 

 These are the conditions under which night journeys are made to avoid 

 the burning sun. The planet Venus, at inferior conjunction especially, 

 sheds light sufficient for the traveler over open country. 



There are at times nights of remarkable luminescence. Clouds 

 become phosphorescent, and often under certain states of electric 

 stress, during high winds, glimmer with a faint light not amounting 

 to a discharge of the electric fluid. Fretpiently successive flashes of 

 "heat lightning" aid the traveler in finding his way. It is possible, 

 also, that the soil over certain regions may become phosphorescent 

 under the light of the sun and retain the property during the night, 

 as certain gems are phosphorescent after l)eing submitted to sunlight. 

 Snow has this property. Gaseous emanations of a phosphorescent 

 character are occasionally abundant enough to })roduce temporary 

 ilhunination. 



Next to the sun in value to man as a light producer is the moon. 

 Though intermittent in the power and duration of its light, the moon 

 has proven a valuable auxiliary' on the night side of man's life, and 

 its period has given a measurement of aggregates of time. 



In torrid climates, and at hot seasons of the year, work is often 



"Read at the Conjrres International d'Anthropologie et d'Archeologie Prehisto- 

 rique.«, XII session, Paris, Au;;ust, 1900. (Reprinted from the American Anthropolo- 

 gist (N. S.), Vol. Ill, April-.lnne, 1901.) 



493 



