The first great discovery in solar physics, by means of the analysis of the prism (though the discovery had little meaning at the time), consisted in the recognition of the fact that, by means of such devices as the above, dark gaps or cross-lines _can_ be seen in the solar spectrum. In other words, light-waves of the various gradations corresponding to all the tints of the spectrum from violet to red do _not_ travel to us from the great central luminary of our system. Remembering that the effect we call color is due to the length of the light-waves, the effect of red corresponding to light-waves of the greatest length, while the effect of violet corresponds to the shortest light-waves, we see that in effect the sun sends forth to the worlds which circle around him light-waves of many different lengths, but not of all. Of so complex and interesting a nature is ordinary daylight. But spectroscopists sought to interpret these dark lines in the solar spectrum, and it was in carrying out this inquiry—which even to themselves seemed almost hopeless, and to many would appear an utter waste of time—that they lighted upon the noblest method of research yet revealed to man. They examined the spectra of the light from incandescent substances (white-hot metals and the like), and found that in these spectra there are no dark lines. They examined the spectra of the light from the stars, and found that these spectra are crossed by dark lines resembling those in the solar spectrum, but differently arranged. They tried the spectra of glowing vapors, and they obtained a perplexing result. Instead of a number of dark lines across a rainbow-tinted streak, they found bright lines of various colors. Some gases would give a few such lines, others many, some only one or two. Then they tried the spectrum of the electric spark, and they found here also a series of bright lines, but not always the same series. The spectrum varied according to the substances between which the spark was taken and the medium through which it passed. Lastly, they found that the light from an incandescent solid or liquid, when shining through various vapors, no longer gives a spectrum without dark lines, but that the dark lines which then appear vary in position, according to the nature of the vapor through which the light has passed. Here were a number of strange facts, seemingly too discordant and too perplexing to admit of being interpreted. Yet one discovery only was wanting to bring them all into unison.