there is the brighter incandescent solid or liquid photosphere of the sun. It is this brighter background that makes the lines black, just as the bars of a window appear dark when viewed against the sky, though painted white. A truer idea is afforded by the illustration of the comb. Let us take, for example, the spectrum of sodium. Here there is only one tooth, all the other primitive colours being wanting. This single tooth is yellow, and we shall suppose made of glass. If you look through this yellow glass at another yellow but much brighter object, such as the yellow portion of the sun's spectrum, it becomes a dark line. The yellow tooth, or bar of sodium, when thus seen against the yellow portion of the sun's spectrum, becomes a black line, and we know that it is the same, from its position in the spectrum. Finding that the dark line, called D, in the solar spectrum, corresponds with the bright yellow band of sodium, we conclude that sodium is part of the gaseous solar atmosphere interposed between us and the sun. Other substances have more bands or teeth; and as there are many substances in the sun's atmosphere, the lines are like so many combs laid the one over the other, each having its own character, number, and disposition of teeth, so that, though they appear confused, it is possible to single out the more marked with great ease. The sailor can easily detect, through the forest of masts in a harbour, the rig of his own
