A VOYAGE IN SPACE
and there are much the same advantages in photographing here as in the case of the direct light.
But we can also do something else. When the light is spread out in this way we can throw most of it away and keep only one colour, and then nothing will shine unless it has that colour. Let us do another experiment or two with our prism. Here is the light spread out into colours as usual, and now I will take a red ribbon and put it in the red light: you see it looks red as usual, but in the green or blue it looks black—it gives us no light. Here is a yellow ribbon which shines yellow in the yellow light, but when I put it in the red or blue it looks black: and so on.
We will try another experiment. Instead of spreading out our light into colours, let us take light of only one colour, to begin with. We can do that by putting some common table salt into the flame of a spirit-lamp, when it will blaze with that brilliant yellow colour which we have seen is due to sodium. Now here is our bunch of ribbons of all colours, which perhaps one of my audience will kindly hold: when we burn our table salt only the yellow ribbon will show—the others will all appear black but we are not quite ready yet. Here are some letters of different colours pasted on this board, which perhaps two others of the audience will hold: and here again is a picture in colours for some one else. You see all these coloured objects in ordinary light: now we will darken the room and put in the table salt, and in the weird yellow light you see only the yellows—all the other colours appear black, but we see at a glance where there is any yellow.