pose we made a letter picture of those last three words—
| UNDERSTAND THE REASON |
E EE |
A A |
| Full Light. | Yellow Light. | Red Light. |
colouring all the letters E with yellow, all the letters A with red, and the other letters green. Then if we took a photograph in yellow light we should get only the E's as in the second diagram; if we took a photograph in red light we should get only the A's. The two photographs would be quite different although we had actually photographed the same picture. Perhaps some of you have already taken colour photographs by the three-colour process; if so you will know about this principle without this explanation. The point is that just as we find out in one photograph where the letters E are distributed and in the other photograph the letters A, so in the case of the Sun we find in one photograph where the calcium is distributed and in the other photograph the hydrogen. Why should these two substances be arranged so differently? We get a hint of one probable reason from Fig. 63, in which we see a number of curved lines. There is no such regularity in the calcium picture; but the hydrogen picture shows these curves, which remind us at once of curves made by iron filings when near a magnet. Let us throw a picture of them on the screen. We first place a bar magnet on this sheet of glass and then
