The smallest portion of light which we can speak of is called a ray of light. You have seen, when what you call a beam of light comes in at a hole, before the shutters have been opened, how the little specks of dust glance up and down in it, as if they were at an endless game of puss-in-the-corner. But have you ever seen beautiful colours, like those of the rainbow, dance about the room--now on the ceiling, now on the floor? You can best see this lovely little rainbow by darkening the room, and letting just one ray of light stream in through a small hole. Then take a bit of glass, cut so that it has at least three sides--a "drop" of cut glass from the lustre on the mantelpiece will do--and hold it up between you and the light. This little piece of glass, which is called a prism, because it has been sawn or cut, will do a wonderful thing, as you turn it about in the sunbeam. The ray of light, as it passes through the three-cornered bit of glass, will be turned out of its straight path, and this causes it to be split up into many colours, so that you will have a tiny rainbow, which can be seen beautifully if you allow it to fall upon a sheet of white paper; and the colours are always arranged in the same way. Look! in the centre of your rainbow there are green and yellow; then comes red, then blue, then violet. You can easily see these five colours; and two more are counted; indigo, or dark blue, and orange. The only difficulty about saying how many colours you can see is this. If you begin with the violet, and count till you come to the red, you will find that the soft hues are so blended, or run into each other, that it is not easy to see where one ends and the other begins. I want you to make this little rainbow, not only because the colours which it paints upon the ceiling are so pure and beautiful, and it is so curious to see the bright band of red and blue and green dancing from place to place as you turn your bit of glass, but because you can see in this way how a ray of light spreads itself out when it passes through this glass with three sides. The colours are separated from each other because no two waves of light are of quite the same length; some move slowly and others fast, and the faster a wave travels the more it is turned aside out of the straight road. This is a difficult subject, but I think you will understand that if all rays were alike, the whole beam would be bent; but as some are more easily bent than others, as they pass through the prism they are spread out. Long ago, the great philosopher Newton bought a prism, and thus "analysed" or broke up the sunbeam, and discovered what is called the "prismatic band" of colours. He found that what seemed to be white light was made up of tints really infinite in number; for though we count only seven prismatic colours, they are shaded off, one into the other, as you see.