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ways. Suppose the particles are being shot up out of the Sun; then they will scatter as they go, like the sparks out of a rocket; and being more diffused they will send us less light per unit area. This will explain in a general way why the corona should be fainter as we recede from the Sun; but in science we must not be content with general explanations, we must see whether they fit particularly. We can calculate not only that the corona ought to be fainter but how much fainter it ought to be; and we can measure on the photographs how much fainter it is; and see whether the two "how much's" agree: if not, then our idea of the particles being shot up won't work and we must try some other idea, perhaps that they are falling continually down. When I tried these different ideas I concluded that the particles were behaving as a fountain behaves—both being shot up and falling down again. It would take us too long to explain why—all I want you to see is the kind of way in which we may learn about the corona from photographs at an eclipse.

But I have not explained why there need be solid particles in it at all. Why should it not be merely a mass of gas like our own air? That again can be answered from the photographs if we take them in what is called polarized light. I am afraid I am worrying you with rather hard notions this afternoon, or at any rate hard names; there is nothing very hard about the notion of polarized light, if you will think of a ray of light as a flat thing like a strip of cardboard which will bend quite easily sideways but not edgeways. I ought, however, to