b, a milled head, with screw motion to adjust the focus of the achromatic eye-lens.

c, milled head, with screw motion to open or shut the slit vertically. Another screw at right angles to c, and which from its position could not be shown in the cut, regulates the slit horizontally. This screw has a larger head, and when once recognised cannot be mistaken for the other.

dd, an apparatus for holding a small tube, in order that the spectrum given by its contents may be compared with that from any other object placed on the stage.

e, a square-headed screw opening and shutting a slit to admit the quantity of light required to form the second spectrum. Light entering the round hole near e strikes against the rightangled prism which we have mentioned as being placed inside the apparatus, and is reflected up through the slit belonging to the compound prism. If any incandescent object is placed in a suitable position with reference to the round hole, its spectrum will be obtained, and will be seen on looking through it.

f shows the position of the field lens of the eyepiece.

is a tube made to fit the microscope to which the instrument is applied. To use this instrument, insert g like an eyepiece in the microscope tube, taking care that the slit at the top of the eyepiece is in the same direction as the slit below the prism. Screw on to the microscope the object-glass required, and place the object whose spectrum is to be viewed on the stage. Illuminate with stage mirror if transparent, with mirror and Lieberkühn and darken well if opaque, or by side-reflector bull's eye, &c. Remove a, and open the slit by means of the milled head, not shown in cut, but which is at right angles to dd. When the slit is sufficiently open, the rest of the apparatus acts like an ordinary eyepiece, and any object can be focussed in the usual way. Having focussed the object, replace a, and gradually close the slit till a good spectrum is obtained. The spectrum will be much improved by throwing the object a little out of focus.

Every part of the spectrum differs a little from adjacent parts in refrangibility, and delicate bands or lines can only be brought out by accurately focussing their own parts of the spectrum.

This can be done by the milled head b. Disappointment will occur in any attempt at delicate investigation if this direction is not carefully attended to.

When the spectra of very small objects are to be viewed, powers of from inch toth or higher may be employed. The prismatic eyepiece is shown in section in Fig. 48.

FIG. 48.

Blood, madder, aniline red, permanganate of po tash solution, are convenient substances to begin experiments with. Solutions that are too strong are apt to give dark clouds instead of delicate absorption bands.

LECTURE V.

Foundation of Solar and Stellar Chemistry.—Examination of the Solar Spectrum.-Fraunhofer, 1814.-Kirchhoff, 1861.-Coincidence of Dark Solar Lines with Bright Metallic Lines.-Reversion of the Bright Sodium Lines.-Kirchhoff's Explanation.-Constituents of the Solar Atmosphere.-Physical Constitution of the Sun.-Planet and Moonlight.

Appendix A.-Lockyer's and Janssen's Discoveries respecting the Solar Prominences. Conclusion deduced therefrom.

Appendix B.-Extracts from the Report of the Council of the Royal Astronomical Society to the 49th Annual General Meeting. Appendix C.-Ångström, Recherches sur le Spectre normal du Soleil.

WE have in this lecture the somewhat formidable task set before us of endeavouring to explain the grounds upon which Professor Kirchhoff concludes with certainty, that in the solar atmosphere, at a distance of about 91 millions of miles, substances such as iron, sodium, magnesium, and hydrogen, which we know well on this earth, are present in a state of luminous gas.

In beginning to consider this matter, we shall, however, do well to remember that the subject is still in its infancy; that it is only within the last few years that we have been at all acquainted with the chemistry of these distant bodies. We must not be surprised to find that some of our questions cannot be satisfactorily answered, and we may expect in several instances to meet with facts to which an explanation is still wanting.

In the first lecture I pointed out to you that sunlight differs from the light given off by solid and liquid substances, as well as from the light given off by gaseous

bodies. If we were experimenting with sunlight now, and if I could throw the solar spectrum on to the screen, instead of this continuous spectrum of the incandescent carbon poles we should find that this bright band was cut up by a series of dark lines or shadows.

These lines I mentioned to you were first discovered in 1814 by Fraunhofer—at least they were first carefully observed by him-and have since gone by the name of Fraunhofer's lines.

Fraunhofer measured the distances (see Fig. 12, p. 23) between these fixed lines, and he found that the distance from D to E, and from E to F, remained perfectly constant in the sunlight, that they are fixed lines which always appear in sunlight; and, moreover, as I think I mentioned to you on a previous occasion, he examined the light from the moon and from the planet Venus, and found that the same lines occur in moonlight and in planet-light, which is simply reflected sunlight, and he found that the relative distances between these lines were the same in light from these three sources. He then examined the light from some fixed stars, from Sirius and others, and he found that, although in some of these fixed stars some lines existed which occur in sunlight, yet that other lines, always present in sunlight, are absent from the light of the stars thus in Procyon and Capella he saw two solar lines D, but other well-known solar lines were wanting.

So long ago as 1814, Fraunhofer concluded that these lines were caused by some absorptive power exerted in the star or in the sun.

The exact mapping of these lines becomes a matter of very great importance, and, since the time of Fraunhofer, the best maps which have been made of these solar lines. are those of Kirchhoff and Ångström. Facsimile drawings

of these maps are, with the permission of the authors, given in Plates III., IV., and V.

I will now project, by means of the oxyhydrogen light, a photograph of one of the diagrams of Professor Kirchhoff upon the screen, and show you the great number of lines existing in the solar spectrum (see Plate III. facing this Lecture). This is the line D in the yellow, which was noticed by Fraunhofer, and observed

by him to be double. Thanks to the kindness of Mr. Browning, I have on the other end of the table a very beautiful instrument, which is so arranged that it enables me to show these double D lines. Reverting again to the map we see a great number of lines varying in intensity, in depth of shade, as well as in breadth: here we come to E in the blue. I might in the same way show you that throughout the whole length of the spectrum similar groups of dark lines occur. of dark lines occur. From these