APPLIED TO THE HEAVENLY BODIES. 197 



may be distinguished from each other by their spectra. Each element, and 

 every compound body that can become luminous in the gaseous state without 

 suffering decomposition, is distinguished by a group of lines peculiar to itself 

 These green lines are produced by silver in a state of gas, and only by silver 

 gas. It is obvious that if the groups of lines characterizing the different terres- 

 trial substances be known, a comparison of these as standard spectra with the 

 spectrum of light from an unknown source will show whether any of these 

 terrestrial substances exist in the source of the light. 



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 by the source of the light. They tell us of vapors 

 through which the light has passed on its way, and which have robbed the light, 

 by absorption, of certain definite colors or rates of vibration; such spectra are 

 form<'d by the light of the sun and stars. 



Kirchhoff has shown that if vapors of terrestrial substances come between the 

 eye and an incandescent body, they cause groups of dark lines, and, further, 

 that the group of dark lines produced by each vapor is identical in the number 

 of the lines and in their position in the spectrum with the group of bright lines 

 of which its light consists when the vapor is luminous. 



Mr. Ladd will throw upon the screen the spectrum of incandescent carbon 

 points which contain scdium. Observe in addition to the continuous spectrum 

 of the incandescent carbon a bright yellow band, which indicates the presence 

 of sodium. Now a piece of metallic sodium will be introduced into the lamp. 

 The sodium will be vaporized by the heat, and will fill the lamp with its vapor. 

 This vapor absorbs, quenches the light that it emits when luminous. There 

 will thus be produced a black line exactly in the place where the bright yellow 

 line was seen. 



It is evident that Kirchhoff by this discovery has furnished ais with the means 

 of interpreting the dark lines of the solar spectrum. For this purpose 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 in the solar spectrum. 

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

 we know that the terrestrial substance 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 absorption, can produce that paiticular 

 group of dark lines. In this way Kirchhoff discovered the presence of several 

 terrestrial elements in the solar atmosphere. 



METHODS OF OBSERVATION. 



I now pass to the special methods of observation by which, in our investiga- 

 tions, we have applied these principles of spectrum analysis to the liglit 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 eye, is the 

 air sufficiently steady for these extremely delicate observations. Further, the 

 light of the star is feeblC: 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 object-glass of eight inches aperttire 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 cai-ries the astronomer and his instruments 

 with it. This movement was counteracted by a movement given by clockwork 

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

 to retain the image of a star for any length of timp. exactly within the jaws of a 

 slit only the l-300th of an inch apart. By patient perseverance these diffipul- 



