. TRANSACTIONS OF WAGNER 



64 



ETHER WAVES AND THE MESSAGES THEY BRING 



If the light from a glowing solid or liquid which, as we have seen, 

 contains all possible wave lengths of Hght, be passed through a gas, it emerges 

 apparently lacking those wave lengths which the gas itself produces. Thus if 

 hydrogen be studied, it will be seen to produce in the spectroscope a certain 

 set of bright lines. If the light from a glowing solid or liquid be passed through 

 hydrogen, it will be seen that there are dark lines just where the bright Hues of 

 the hydrogen spectrum were. Such a spectrum is called an absorption or dark 

 line spectrum (Fig. i, 5). It is found that these dark lines are dark only in 

 comparison with the light areas produced by the solid ; for the waves produced 

 by the gas are there, but the gas, being at the lower temperature, produces 

 wave trains not nearly so intense as those it has absorbed. 



The three kinds of spectra possible, then, are: continuous ones, formed by 

 the light from a glowing liquid or soUd ; bright line spectra, from glowing gases ; 

 and dark line spectra, formed by passing light from a glowing Hquid or solid 

 through a gas, the gas being usually at a lower temperature. 



Much of our knowledge of the composition of the sun and stars is based 

 on the facts just set forth. You see on the screen a representation of the 

 spectrum formed by sunlight (Fig. i, 5). Notice that it is a dark line spec- 

 trum. The spectra of the stars are of the same sort. We conclude, therefore, 

 that the sun and stars are solid or hquid masses, glowing hot, surrounded by 

 atmospheres of gaseous material. The spectra of the nebulae are entirely 

 different, being bright line spectra. This shows that the nebulae are masses of 

 gaseous material without any solid or hquid core. In the Milky Way in some 

 places it is impossible to tell, even with the most powerful telescope, whether 

 the luminous areas are multitudes of stars set so thickly that the telescope will 

 not separate their individual images, or whether they are enormous masses of 

 incandescent gases. The spectroscope settles the matter instantly. Turned 

 upon some of these luminous areas we get a dark line spectrum, indicating 

 vast masses of stars each having a central core of solid or liquid material. 

 From others we get a bright line spectrum indicating the presence merely of 

 vast bodies of incandescent vapors. 



If we study carefully the solar spectrum, we find that it presents a very 

 large number of dark hnes. Fraunhofer, who first made a careful study of these 

 lines, counted and mapped about 700 of them. They are called after him, 

 Fraunhofer's lines. Many more have been discovered since his time, there 

 being now about 12,000 of them which are not identified with the known lines 



