234 ANNUAL OF SCIENTIFIC DISCOVF,RT. 



such a spectrum we may learn much. It informs us that the lumi- 

 nous matter from wliich the lij^lit has come is in the slate of gas. 

 It is only when a luniiiioiis l)()dy is free from the molecular tram- 

 mels of solidity anil liiiuidily, tliat it can exhibit its own jieculiar 

 power of rudiatini^ some colon-il rays alone. Hence substances, 

 when in a state of <^as, may be distinguished from each other by 

 their spectra. Each element, and every compound body tiiat can 

 become luminous in ilw. gaseous state with(Mit suffering decompo- 

 sition, is di.^tinguisiied \>\ a group of lines {.eculiar to itself. 



" 3. The third order consists of the spectra of incandescent solid 

 or liquid bodies, in which the continuity of the colored light is 

 broken by dark lines. These dark spaces are not produced In' the 

 souri'e of the liglit. They tell us of vapors through wiiieh the 

 light has j)asscd on its way, and whicii liave robbed the liglit, Ijy 

 absorption, of certain definite colors or rates of vibration. Such 

 spectra are formed by the light of the sun and stars. 



*' Kireliholf has shown that, if vapors of terrestrial substances 

 come between the ej'e and an incandescent body, they cause 

 groups of dark lines; and, further, that tlu; group of dark lines 

 produced by each vapor is identical in the numl)er of the lines and 

 in their position in tlu; spectrum witii the group of bright lines of 

 which its light consists when the vapor is luminous. 



" It is evident that Kirchhoff, by this discovery, has furnished us 

 with the means of interi>reting the dark lines of the solar spec- 

 trum. For this piu'i)ose it is necessary to compare the bright lines 

 in the spectra of the light of terrestrial substances, when in the .state 

 of gas, with the dark lines of the .solar spectrum. When a group 

 of bright lines coincides with a similar group of dark lines, then 

 we know that tiie terrestrial sul)stance producing the bright lines 

 is present in the atmosphere of the sun; for it is this substance, 

 and this substance alone, which, by its own peculiar power of ab- 

 sorption, can produce that particular group of dark lines. In this 

 way Kirchhoff discovered the presence of several terrestrial ele- 

 ments in the solar atmosphere. 



^'Methods of Obserration. — I now pass to the special methods 

 of observation by which, in our investigations, we have applied 

 these principles of spectrum analysis to the light of the heavenly 

 bodies. I may here state that several circumstances unite to 

 make these observations very difficult and very irksome. In our 

 climate, on few only even of those nights in which the stars shine 

 brilliantly to the naked ej^e, is the air sufficiently steady for these 

 extremely delicate observations. Further, the light of the .stars 

 is feelde. This difficulty has been met, in some measure, by the 

 employment of a large telescope. The light of a star falling 

 upon the surface of an ol:)ject-glass of eiglit inches aperture is 

 gathered up and concentrated at the focus into a minute and 

 brilliant point of light. 



" Another inconvenience arises from the apparent movement of 

 the stars, caused by the rotation of the earth, which carries the 

 astronomer and his instruments with it. This movement was 

 counteracted by a movement given, h^ clockwork, to the telescope, 

 iu the opposite direction. In practice, however, it is not easy to 



