Next, this important consideration comes into play—whenever any element finds itself in this state of fineness, and therefore competent to give rise to these phenomena, it will give rise to them in different degrees according to certain conditions. The intensest form is observed when we employ electricity. In a great many cases the vibrations may be rendered very intense by heat. The heat of a furnace or of gas will, for instance, in a great many cases, suffice to give us these phenomena; but to see them in all their magnificence—their most extreme cases—we want the highest possible temperatures, or better still, the most extreme electric energy. What we get is the vibration of these particles rendered visible to our eye by the bright images of the slit or by their bright “lines.” But that is not the only means we have of studying these states of unrest. We can study them almost equally well if, instead of dealing with the radiation of light from the particles themselves, we interpose them between us and a light source of more complicated molecular structure, and hotter or more violently excited than the particles themselves. From such a source the light would come to us absolutely complete; that is to say, a perfectly complete gamut of waves of light, from extreme red to extreme violet. When we deal with these particles between us and a light-source competent to give us a continuous spectrum, _then we find that the functions of these molecules are still the same, but that their effect upon our retinas is different_. They are not vibrating strongly enough to give us effectively light of their own, but they are eager to vibrate, and, being so, they are employed, so to speak, _in absorbing the light which otherwise would come to our eyes_. So that whether we observe the bright spectrum of calcium or any other metal, or the absorption spectrum under the conditions above stated, we get lines exactly in the same part of the chromatic gamut, with the difference that when we are dealing with radiation we get bright lines, and when dealing with absorption we get dark ones. It was such considerations as these by which the presence of sodium was determined in the sun. Soon followed the discovery of coincidence of other dark lines with the bright lines of numbers of our elements, and we had maps made by Kirchhoff, and Bunsen, and Ångström, in which almost every dark line is mapped with the greatest accuracy. The dark lines in the spectra of the stars, and the light ones in nebulæ, comets, and meteorites have also yielded to us a knowledge more or less accurate of the elements of which these celestial bodies are built up.