determination of the composition of the photosphere or mass of luminous vapour surrounding the body of the sun.
A simple experiment will show how this brilliant discovery was arrived at. The light of a candle or other flame containing incandescent solid matter is passed through the spectroscope, and is found to decompose into a continuous spectrum, uninterrupted by dark lines. Between the light and the slit a spirit-lamp is placed, but no difference in the appearance of the spectrum is perceived. Introduce, however, the smallest portion of a soda salt into the non-luminous flame of the second-lamp, and a broad black line is immediately seen, crossing the middle of the yellow portion of the band of colour. Remove the sodium flame and the band disappears; but do the same with the lamp producing the spectrum, and the spectrum of course disappears, and the dark band caused by the sodium flame is changed to the yellow line produced by that metal. The same experiments may be tried with potassium, strontium, and other metals; and we shall always find that when a coloured flame is introduced between an incandescent solid and its continuous spectrum, it produces a series of black lines corresponding to the substances by which it is coloured. Thallium, in like manner, would give a black band in the middle of the green, and indium a similar one in the indigo. (Fig. 6, Frontispiece.)
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 so-
