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Rh or brighter. But it fades away so rapidly that at about a diameter from the Sun it is very difficult to see it or photograph it. When it is photographed you get dense blackness on the negative for the bright parts and hardly any effect for the faint parts. My work was to measure just how dense the different parts were; and the measurements are best made when the grades are not either too dense or too faint, but intermediate. Hence, you see why it is desirable to take photographs with different exposures; for by giving a long exposure you can make the fainter portions affect the plate more; it is as though they were not so faint. On the other hand, the short exposure of one second will suit the dense parts better.

But supposing these measures made, and the brightness of different parts compared, what then? What use will that be?

Well, we want to find out what the corona is made of; is it simply an atmosphere of gases like our own round the Earth, or has it solid particles in it, as our own atmosphere sometimes has in a fog? And again, is the atmosphere at rest or moving? If there are solid particles in it are they floating quietly, or are they being shot up out of the Sun with great velocity? Or are they, perhaps, falling continually into the Sun? If an eclipse only lasted long enough, we might perhaps answer some of these questions by watching the changes; but in the few minutes of a total eclipse the changes are too small to be noticed, and, by the time the next eclipse comes, everything has been altered past recognition. Hence, we have to work in other