Some very interesting observations have also been made by Bunsen upon the absorption spectrum of didymium,1 from which we learn that the didymium spectrum, and also that of erbium, undergo changes if examined by polarized light according as the ordinary or extraordinary ray be allowed to pass through the crystal. These changes only become visible, however, when a powerful battery of prisms and a telescope of high magnifying power are employed. According to the direction in which the ray of polarized light is allowed to traverse the crystal of didymium sulphate is the position of the dark absorption bands found to vary; whilst the bands produced by the solution of the salt in water are again different. Very remarkable are the small alterations in the position of the dark bands of the didymium salts, dependent upon the nature of the compound in which the metal occurs. These changes are too minute to be seen with a small spectroscope, but are distinctly visible in the larger instrument. "The differences thus observed in the absorption spectra of different didymium compounds cannot, in our complete ignorance of any general theory for the absorption of light in media, be connected with other phenomena. They remind one of the slight gradual alteration in pitch which the notes from a vibrating elastic rod undergo when the rod is weighted, or of the change of tone which an organ-pipe exhibits when the tube is lengthened."

From experiments made with erbia and other earths by Huggins and Reynolds, they conclude that the bright lines seen in the spectra obtained by heating these earths in the oxyhydrogen flame are due to the partial vaporization of the heated substance. If this is really the case, the only exception to the law of solid bodies giving continuous spectra will disappear.

1 Phil. Mag. vol. xxxii. 126, p. 177.

2 Proc. Roy. Soc. xviii. 546.

'APPENDIX G.

DESCRIPTION OF THE SORBY-BROWNING MICRO-SPECTROSCOPE.

The construction of this instrument is represented in Figs. 50 (page 183) and 55. The prism is contained in a small tube (a),

which can be removed at pleasure, and which is shown in section in Fig. 51. Below the prism is an achromatic eyepiece, having an adjustable slit between the two lenses; the upper lens being furnished with a screw motion to focus the slit. A side slit capable of adjustment admits when required a second beam of light from any object whose spectrum it is desired to compare with that of the object placed on the stage of the microscope. This second beam of light strikes against a very small prism, suitably placed inside the apparatus, and is reflected up through the compound prism, forming a spectrum in the same field with that obtained from the object on the stage.

a is a brass tube carrying the compound direct-vision prism. 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 recognized cannot be mistaken for the other.

d d, 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.

g 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 d d. 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 tooth or higher may be employed. The prismatic eyepiece is shown in section in Fig. 51.

Blood, madder, aniline red, and permanganate of potash 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.-Lockyer and Janssen's Researches.— Phenomena observed during Total Eclipses. -Constitution of the Red Solar Prominences; their gaseous Nature and rapid Motion. -The Chromosphere.-Physical Constitution of the Sun.-The Corona. Sun-spots and Faculæ,

APPENDIX A.-"Recherches sur le Spectre normal du Soleil," by A. J. Ångström.

APPENDIX B.-The Total Solar Eclipses of August 18, 1868, of December 22, 1870, and of December 11, 1871.-Extracts from Reports of the Council of the Royal Astronomical Society. APPENDIX C.-Spectroscopic Observations of the Sun, being abstracts of the various Papers on this subject by Lockyer, Janssen, Huggins, and Zöllner.

APPENDIX D.-Preliminary Catalogue of Bright Lines in the Chromosphere, by Professor C. A. Young.

APPENDIX E.—Professor Balfour Stewart on the Equality of Radiation and Absorption.

APPENDIX F.-On the Harmonic Ratios observed in the Wave-lengths of the Lines of certain Spectra.

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,