The exact position of the black band in the middle of the yellow is shown in the coloured figure of the spectrum so beautifully printed in the frontispiece of this book, and it has been found to correspond exactly with the dark line D of the solar spectrum. The inference from this fact is obvious. The incandescent portion of the sun gives off light corresponding in its properties to that emitted by the solid matter contained in the candle flame, but the photosphere containing the vapour of sodium cuts off that portion corresponding to the sodium line. Accurate measurements prove that numberless other lines occurring in the solar spectrum are due to the vapours of other well known metals existing on the earth. Amongst these may be mentioned potassium, calcium (the base of lime), iron, nickel, chromium, and several others. This discovery with regard to the sun has resulted in the spectral examination of a large number of the fixed stars and nebulæ. For centuries the fixed stars refused to answer all questions put to them by mortals. The telescope showed them merely as bright points. Their nature and origin remained a beautiful mystery, until Dr. Miller, Mr. Huggins, Father Secchi, and a few other philosophers interrogated them in a manner that could not fail to draw forth an answer. They brought their light within range of their prisms, and forthwith they declared themselves to be suns like our own. It is true that before this they were looked on by most astronomers as bodies analogous to our own sun, but it was only reasoning from analogy, after all; but we are now able to assert with all the certainty that is compatible with human fallibility that many of these heavenly bodies are possessed of an incandescent centre, surrounded by a photosphere or envelope of gaseous matter in a luminous condition. It would be impossible to give a list of all the stars that have been examined up to the present time; the composition of the photospheres of a few must therefore suffice. It is singular that the elements hitherto discovered in the stars are those which are more or less abundant on the earth. Amongst them we may name hydrogen, nitrogen, sodium, magnesium, barium, iron, antimony, bismuth, tellurium, and mercury. The bright star in the constellation of Orion known as Betelgeux is one of the most singular in composition, the lines of its spectrum indicating the absence of hydrogen. If, as Messrs. Huggins and Miller suggest, the worlds revolving round this star are also deficient in this element, they would be without water, like our moon.