of the Sun, where we see great red flames shooting up to enormous heights. Perhaps we ought not to call them flames, because in our earthly fire-grates a flame means that something is being burnt, generally the gas from the coal which is being burnt up in the air. The solar flames do not represent anything burning; they glow like the filament of an electric lamp, but we have no better word to describe them than "flames."
We cannot see these flames in the ordinary way; but because they are of a special red colour we can see them by means of a trick. You remember our experiments with colours two lectures ago: we found that a red ribbon would show bright in the red, but in green light it appeared black; and do you remember how we lighted the room with the yellow light made by burning common salt, and then only yellow things showed bright? All other colours in a picture we were looking at—the reds and blues and violets—all disappeared. If we had taken a photograph in this yellow light, the yellow parts of the picture would have shown up, while the others would have been quite faint or altogether absent. And I then told you that this trick was used to take photographs, with an instrument called the spectro-heliograph, and with this instrument we can photograph the red flames round the Sun's edge; of course we use red light in order to show them up. The general plan is simply this: if we pass the Sun's light through a glass prism we have seen that it is spread out into all the different colours. Now let us block out all the other colours except the special red we want; for
