efore beginning upon the subject of our lecture to-day I want to tell you the story of a great puzzle which presented itself to me when I was a very young child. I happened to come across a little book—I can see it now as though it were yesterday—a small square green book called World without End, which had upon the cover a little gilt picture of a stile with trees on each side of it. That was all. I do not know what the book was about, indeed I am almost sure I never opened it or saw it again, but that stile and the title "World without End" puzzled me terribly. What was on the other side of the stile? If I could cross over it and go on and on should I be in a world which had no ending, and what would be on the other side? But then there could be no other side if it was a world without any end. I was very young, you must remember, and I grew confused and bewildered as I imagined myself reaching onwards and onwards beyond that stile and never, never resting. At last I consulted my greatest friend, an old man who did the weeding in my father's garden, and whom I believed to be very wise. He looked at first almost as bewildered as I was, but at last light dawned upon him. "I tell you what it is, Master Arthur," said he, "I do not rightly know what happens when there is no end, but I do know that there is a mighty lot to be found out in this world, and I'm thinking we had better learn first all about that, and perhaps it may teach us something which will help us to understand the other."
I daresay you will wonder what this anecdote can have to do with a lecture on the sun—I will tell you. Last night I stood on the balcony and looked out far and farther away into the star-depths of the midnight sky, marvelling what could be the history of those countless suns of which we see ever more and more as we increase the power of our telescopes, or catch the faint beams of those we cannot see and make them print their image on the photographic plate. And, as I grew oppressed at the thought of this never-ending expanse of suns and at my own littleness, I remembered all at once the little square book of my childish days with its gilt stile, and my old friend's advice to learn first all we can of that which lies nearest.
So to-day, before we travel away to the stars, we had better inquire what is known about the one star in the heavens which is comparatively near to us, our own glorious sun, which sends us all our light and heat, causes all the movements of our atmosphere, draws up the moisture from the ground to return in refreshing rain, ripens our harvests, awakens the seeds and sleeping plants into vigorous growth, and in a word sustains all the energy and life upon our earth. Yet even this star, which is more than a million times as large as our earth, and bound so closely to us that a convulsion on its surface sends a thrill right through our atmosphere, is still so far off that it is only by questioning the sunbeams it sends to us, that we can know anything about it.
You have already learnt[1] a good deal as to the size, the intense heat and light, and the photographic power of the sun, and also how his white beams of light are composed of countless coloured rays which we can separate in a prism. Now let us pass on to the more difficult problem of the nature of the sun itself, and what we know of the changes and commotions going on in that blazing globe of light.
We will try first what we can see for ourselves. If you take a card and make a pin-hole in it, you can look through this hole straight at the sun without injuring your eye, and you will see a round shining disc on which, perhaps, you may detect a few dark spots. Then if you take your hand telescopes, which I have shaded by putting a piece of smoked glass inside the eye-piece, you will find that this shining disc is really a round globe, and moreover, although the object-glass of your telescopes measures only two-and-a-half inches across, you will be able to see the dark spots very distinctly and to observe that they are shaded, having a deep spot in the centre with a paler shadow round it.
Fig. 45.
Face of the sun projected on a sheet of cardboard C.
T, Telescope. f, Finder. og, Object-glass. ep, Eye-piece. S, Screen shutting off the diffused light from the window.
As, however, you cannot all use the telescopes, and those who can will find it difficult to point them truly on to the sun, we will adopt still another plan. I will turn the object-glass of my portable telescope full upon the sun's face, and bringing a large piece of cardboard on an easel near to the other end, draw it slowly backward till the eye-piece forms a clear sharp image upon it (see Fig. 45). This you can all see clearly, especially as I have passed the eye-piece of the telescope through a large screen s, which shuts off the light from the window.