324 PROGKERS Tisr PHYSIOS. 



184U. Fouoault WHS perhaps a year later in g-ettino- ivsnlts. 1mt his 

 method is general!}' considered the best. Both uietliods have ])een 

 used by other experimenters, and very important improvements 

 in Foucault's method were made in the United States about 1878. 

 Micheison's method increased enormousl}' the precision of the 

 measurements, and it has been applied by him and by Newcoml), not 

 only for the better determination of the velocity of light in air, but 

 for the solution of many other related problems of first importance. 

 Micheison's final determination of the absolute velocity of light (in 

 the ether) is everywhere accepted as authoritative. 



Another discovery in optics, entirelv accomplished during the nine- 

 teenth centur}" and of the very first importance, is generally known as 

 spectrum analysis. This discovery has not yet ceased to excite admi- 

 ration and even amazement, and especially among those who best 

 understand it. B3" its use hitherto unknown substances have become 

 known; to the physicist it is an instrument of research of the greatest 

 power, and, perhaps more than anything else, it promises to throw 

 light on the ultimate nature of matter; to the astronomer it has 

 revealed the composition, physical condition, and even the motions of 

 the most distant heavenly bodies, all of which the philosophy of a 

 hundred years ago would have pronounced absolutely impossible. 



The beginning of spectrum analysis was in 1802, when an English- 

 man, Dr. Wollaston, observed dark lines interrupting the solai- spec- 

 trum when produced by a good prism upon which the sunlight fell 

 after passing through a narrow slit. A])Out ten years later Fraun- 

 hofer, of Munich, a skillful worker in glass and a keen observer, dis- 

 covered in the spectrum of light from a lamp two yellow bands, now 

 known as the sodium or "D" lines. Combining the three essential ele- 

 ments of the modern spectroscope — the slit, the prism, and the observ- 

 ing telescope — he saw in the spectrum of sunlight "an almost countless 

 number of dark lines." He was the first to use a grating for the pro- 

 duction of the spectrum, using at first fine wire g'ratings and after- 

 wards ruling fine lines upon glass, and with these he made the first 

 accurate measures of the length of light waves. He did not, however, 

 comprehend the full import of the problem which he thus brought to 

 the attention of physicists. Al)Out twenty years hiter Sir flohn Her- 

 schell studied the bright-line spectra of ditt'ei-ent substances and found 

 that they might be used to detect the presence of minute quantities of 

 a substance whose spectrum was known. Wheatstone studied the 

 spectrum of the electric arc passing between metals, and in 1847 Dr. 

 J. W. Draper published a very important paper on the spectra of 

 solids with increasing temperature. Although quite in the dark as to 

 the real nature of the phenomena with which they were dealing, these 

 observers paved the way for the splendid work of the two Germans, 

 Kirchofi' and Bunseu, who, about 1860, found the key to this wonderful 



