274 LIGHT AND ITS ARTIFICIAL PRODUCTION. 



light and color. Color and musical pitch are therefore purely subjec- 

 tive phenomena. 



If the air is not in vibration no sensation of sound can arise, and 

 similarly if the ether is not in vibration then there is no light. A 

 tuning fork which makes as few as one hundred vibratious per second, 

 affects the auditory nerves. But at least four hundred billion ether 

 vibrations per second are necessary to produce the sensation of sight. 

 We can form no conception of this extremely rapid vibration nor of 

 the immense velocity of its propagation. 



The lightning flashes, and in one-seventh of a second its light travels 

 more than 26,000 miles, a distance greater than the circumference of 

 the earth; in eight minutes it would reach the sun. 



Bodies which have the property of emitting light by their own 

 energy are generally called self-luminous bodies. To-day we shall con- 

 sider only the terrestrial sources of illumination whose light we let 

 shine as soon as that source of all light and being, the sun, turns aside 

 its bright and benignant rays. As soon as the great sun sinks like a 

 red ball of fire below the horizon the suns of other planetary systems, 

 the stars, begin to twinkle and send their pale light to the earth. 

 Their rays, tired and weak from their endless journey, tell us the hap- 

 penings of many years ago. If Sirius should be extinguished today 

 its light would nevertheless long continue shining brightly, for it takes 

 years for a ray of light to journey to our earth from that star. Star- 

 light can therefore be of no practical importance, and dark are tlie 

 nights that the moon does not wish us well, reflecting graciously a few 

 of the rays of the sun. But even she, the ever-faithful and true com 

 panion of the earth, fails to completely satisfy our longing for more 

 light. The striving of man to lengthen the days and shorten the 

 nights can ouly be fulfilled by his own energies, by exchanging philo- 

 sophical speculations for a study of reality, and by making use of 

 experimental research once so despised. How successfully this has 

 been accomplished this brilliantly illumined hall gives the plainest 

 testimony. 



It, however, took many centuries to enable man to surround^ himself 

 with such, a dazzling flood of artificial light. Still, in the dim antiquity, 

 as the book of invention records, the Persians, the Medes, the Assyr- 

 ians, and Egyptians illuminated their temples, their palaces, their 

 plazas, and streets in luxurious prodigality. In Memphis, Thebes, 

 Babylon, Susa, and Nineveh they are said to have hardly known the 

 difference between day and night. Along the streets there stood rows 

 of bronze or stone vases filled with as much as 100 pounds of oil, whicli 

 burned from a wick 3 inches in diameter. If such a long-buried civili- 

 zation of the distant East could develop such a dazzling brightness, 

 how far back must be the day that man learned for the first time to 

 kindle the " divine spark." As valuable as time may appear to modern 

 civilization, the amount of it at the disposal of science in the interpre- 

 tation of physical phenomena is practically unlimited. For this reason 



