Crossing the solar spectrum are hundreds and hundreds of dark lines. These could not at first be explained, because this fact of discriminative absorption was not known. We understand now. The sun's white light comes from the photosphere, but between us and the photosphere there is, as we have seen, another solar envelope of relatively cooler vapours--the reversing layer. Each constituent element in this outer envelope stops its own kind of light, that is, the kind of light made by incandescent atoms of the same element in the photosphere. The "stoppages" register themselves in the solar spectrum as dark lines placed exactly where the corresponding bright lines would have been. The explanation once attained, dark lines became as significant as bright lines. The secret of the sun's composition was out. We have found practically every element in the sun that we know to be in the earth. We have identified an element in the sun before we were able to isolate it on the earth. We have been able even to point to the coolest places on the sun, the centres of sun-spots, where alone the temperature seems to have fallen sufficiently low to allow chemical compounds to form. It is thus we have been able to determine what the stars, comets, or nebulae are made of. A Unique Discovery In 1868 Sir Norman Lockyer detected a light coming from the prominences of the sun which was not given by any substance known on earth, and attributed this to an unknown gas which he called helium, from the Greek _helios_, the sun. _In 1895 Sir William Ramsay discovered in certain minerals the same gas identified by the spectroscope._ We can say, therefore, that this gas was discovered in the sun nearly thirty years before it was found on earth; this discovery of the long-lost heir is as thrilling a chapter in the detective story of science as any in the sensational stories of the day, and makes us feel quite certain that our methods really tell us of what elements sun and stars are built up. The light from the corona of the sun, as we have mentioned indicates a gas still unknown on earth, which has been christened Coronium. Measuring the Speed of Light