178 THE SUN — ITS CHEMICAL ANALYSIS. 



Beveral thousands of dark lines are distinguishable in the solar 

 spectrum. 



A phenomenon having been once recognized, it remains for reason 

 to interpret it. How are we to understand this fact, that the light 

 includes obscure spaces, and that when the white ray spreads out into 

 the iridescent band, there should be vacuities in the series of colored 

 rays ? Each of these rays has a power of refraction proper to itself; 

 it is by virtue, indeed, of this difference of refraction that the white 

 light is decomposed in the prism; but how is it that rays of a certain 

 refrangibility are deficient, Avhile those whose refrangibility differs 

 but by an infinitely small amount, more or less, manifest themselves ? 

 The light which we call white — would thus seem, if we might so 

 speak, not to be complete light ? Has it not lost something in coming 

 from its focus to the eye, either in the sun itself, or in the terrestrial 

 atmosphere? It is certain that the light loses some of its rays in 

 traversing the a3rial envelope of our planet. Sir David Brewster 

 first brought this to notice; he showed that new black lines appear 

 in the solar spectrum when the sun approaches the horizon, because 

 the luminous rays pass through a greater space in the atmosphere 

 before reaching us. These dark lines, however, which are due to the 

 movement of the sun, are to be carefully distinguished from the in- 

 variable, normal lines which always show themselves, whatever may 

 be the altitude of the sun in the sky. If the first are to be explained 

 by atmospheric absorption, the second can only be owing to an ab- 

 sorption which takes place at the sun itself. 



The explanation of the dark lines bv an absorption of rays in the 

 solar atmosphere was proposed in 1847 by M. A. Mathiessen in a com- 

 munication made to the Academy of Sciences of Paris. MM. Brews- 

 ter and Gladstone both adopted it ; the former suggesting at the 

 same time a means of verifying this hypothesis. According to the 

 English physicist, if the lines are due to tlie absorbing power of the 

 solar atmosphere, which would arrest certain luminous rays in pref- 

 erence to others, the spectrum ought to be more furrowed with the 

 dark lines in proportion as the rays which produce it issue from 

 points nearer the edge of the solar disc; admitting that the rays 

 from the edge traverse the solar atmosphere through a greater space 

 than those which emanate from the centre. M. Kircholf points out, 

 with good reason, that this inference would be unavoidable if the 

 atmosphere of the sun were inconsiderable in comparison with the 

 diameter of that body; but everything leads to the belief that the 

 sun's atmosphere has, on the contrary, an immense extent, and in 

 that case two rays, issuing one from the. edge, the other from the 

 centre of the luminous disc, would traverse spaces nearly equal before 

 reaching our eye. We should not, therefore, expect to observe any 

 great difference between the spectrum obtained by means of one or 

 the other of these rays. The idea thus thrown out by M. Brewster 

 has never been experimentally verified. 



The phenomenon of the lines remained therefore unexplained; nor 

 would the mysterious reasons, doubtless, have ever been penetrated, 

 had not physicists thought of tudying other spectrums than that of 



