spectrum disappears at the time when, as analysis shows, the carbon is completely removed from the iron, and the manganese is not. Nevertheless, simultaneous comparison of the Bessemer spectrum with the spectrum of manganese oxide, shows the coincidence in a manner so entire as to completely convince anyone who sees it, of the identity of the two spectra. This spectrum of manganese oxide is obtained faintly by bringing manganese chloride into a Bunsen flame; but best and very brilliantly by bringing manganese chloride, pyrolusite or any manganese compound, into the oxyhydrogen flame. Why this spectrum, which is not that of carbon, should disappear at the exact moment when all the carbon is burnt out, is very difficult to understand. Possibly, the point which it is necessary to hit is not that of the complete removal of carbon, but that at which the injurious oxidation of iron begins, viz. when the quantity of manganese becomes too small to combine with the excess of oxygen.

APPENDIX F.

ON THE SPECTRA OF ERBIUM AND DIDYMIUM AND THEIR

COMPOUNDS.

Bunsen1 has shown that the rare earth erbia is distinguished from all other known substances by a peculiar optical reaction of the greatest interest. This solid substance when strongly heated in the non-luminous gas-flame gives a spectrum containing bright lines, which are so intense as to serve for detecting this substance. This singular phenomenon does not, however, constitute any exception to the law of exchanges; for Bunsen has shown that the bands of maximum intensity in the emission spectrum of erbia coincide exactly in position with the bands of greatest darkness in the absorption spectrum. A similar inversion of the didymium absorption bands has also been observed by Bunsen.2

1 Ann. Ch. Pharm. cxxxvii. p. 1.

2 Ibid. cxxxi. p. 255; Phil. Mag. vol. xxviii. p. 246.

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.

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.

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.

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.