Kirchhoff's retina, whereas the photograph gives us the variations of the chemically active rays, indicated by decomposition of silver salt and subsequent development of the image.

Having fully mastered the facts concerning the composition of sunlight, I must now ask you to pass on to the examination of the first of Kirchhoff's discoveries by which the cause of these singular dark solar lines is explained. So long ago as 1814 Fraunhofer discovered that the dark lines D in the sunlight were coincident with the bright sodium lines. The fact of the coincidence of these lines is easily rendered visible if the solar spectrum is allowed to fall into the upper half of the field of our telescope, while the sodium spectrum occupies the lower half. The bright lines produced by the metal, as fine as the finest spider's web, are then seen to be exact prolongations, as it were, of the corresponding solar lines.

These facts, however, remained altogether barren of consequences, so far as regards the explanation of the phenomena, except to the bold minds of Ångström, Stokes, and William Thomson; the last two of whom combining the facts with an ill-understood experiment of Foucault's made in 1849, foresaw the conclusion to which they must lead, and expressed an opinion which subsequent investigations have fully borne out. Clear light was, however, thrown upon the subject by Kirchhoff, in the autumn of 1859.1 Wishing to test the accuracy of this asserted coincidence of the bright sodium line and the dark solar lines with his very

1 Berlin. Akad. Bericht. 1859, 662; Phil. Mag. Fourth Series, xix. 193, xx. 1.

delicate instrument, Professor Kirchhoff made the following very remarkable experiment, which is memorable as giving the key to the solution of the problem concerning the presence of sodium and other metals in the sun. "In order," says Kirchhoff, for I will now give his own words, "to test in the most direct manner possible the frequently asserted fact of the coincidence of the sodium lines with the lines D, I obtained a tolerably bright solar spectrum, and brought a flame coloured by sodium vapour in front of the slit. I then saw the dark lines D change into bright ones. The flame of a Bunsen's lamp threw the bright sodium lines upon the solar spectrum with unexpected brilliancy. In order to find out the extent to which the intensity of the solar spectrum could be increased without impairing the distinctness of the sodium lines, I allowed the full sunlight to shine through the sodium flame, and to my astonishment I saw that the dark lines D appeared with an extraordinary degree of clearness.

"I then exchanged the sunlight for the Drummond's or oxyhydrogen lime-light, which, like that of all incandescent solid or liquid bodies, gives a spectrum containing no dark lines.

"When this light was allowed to fall through a suitable flame coloured by common salt, dark lines were seen in the spectrum in the position of the sodium lines.

"The same phenomenon was observed if, instead of the incandescent lime, a platinum wire was used, which being heated in a flame was brought to a temperature near its melting-point by passing an electric current through it. The phenomenon in question is easily explained upon the supposition that the sodium flame absorbs rays of the same degree of refrangibility as

those it emits, whilst it is perfectly transparent for all other rays."

Kirchhoff had in fact, as far as he had gone, produced artificial sunlight, because he had obtained the two double dark lines in his continuous spectrum. I will try to show the formation of the dark lines of the sodium: for this purpose we will again employ our electric lamp, and I will throw the continuous spectrum of the carbon points on to the screen, and then I will bring into the lower carbon, which is shaped like a cup, a small quantity of metallic sodium and we shall thus see that the vapour of the sodium has the power of absorbing the

FIG. 62.

particular kind of light which it emits, and that in place of the bright sodium line we shall have a dark line. There you observe the dark sodium line. As a further illustration I have here a diagram (Fig. 62) representing what is seen when we look at the spectrum of burning sodium with an instrument such as that which Kirchhoff used. At the bottom (No. 2) we have a drawing of the ordinary sodium spectrum, giving us these bright yellow double lines on a dark background, and above (No. 1) we see a drawing of the spectrum of burning sodium. Instead of two bright yellow lines, we here find we have

two intensely black lines upon a bright continuous spectrum, the "D" light having been absorbed by the sodium vapour. The difference between the intensities of the lights on each side of these lines and in that particular part where the lines fall is so great as to give an actual shadow, which we see as a black line. There is a wellknown experiment by which we cast a shadow with a

luminous object, such as a candle flame: so here, although these black lines are not wholly devoid of light, yet the light is so much less intense than in the surrounding parts, that they appear black to us.

I can illustrate this to you in another way. Here (Fig. 63) we have a large sheet of non-luminous gas flame (bb) burning under a tall chimney (c), and the flame I can

colour by sodium. In front of this I am going to ignite a flame of hydrogen (a), and I will also place in the hydrogen flame some sodium compound; so that we shall have two sodium flames burning, one in front of the other. I want you to notice that the yellow rays passing from this large flame at the back through the hydrogen flame tinged with soda will be absorbed, and that the outer rim of this hydrogen flame, where the temperature is decidedly low, will appear dark; in fact, it will look just as if the hydrogen flame was smoky,—as though we had a smoky candle burning in front of the large flame. There is no carbon in this flame to produce a smoky appearance. We shall have nothing but pure hydrogen burning. We will light our hydrogen here, but before you can see the phenomenon of absorption I must first make a large soda flame. This I do by burning a little sodium, the fumes of which I waft into the flame. Now you see the large flame has turned yellow, and you will notice that in front we get a smoky flame. It is now very distinct. If, instead of a sodium compound, I next place some lithium salt in the flame, no black rim will appear. We shall get the red colour of the lithium flame, but it will not give us any black shadow, because it has no power of absorbing the yellow light. Hence we conclude that the smoky appearance was really caused by the absorption of the yellow "D" light by the sodium vapour in a state of incandescence.

Here is another most ingenious apparatus devised by my friend Professor Bunsen, for exhibiting a constant black sodium flame absorbing the rays of the same degree of refrangibility as it emits. The little cap of yellow flame (d) which floats from the first burner in front of the larger yellow soda flame (g) absorbs the "D" rays,