tate, dissolved in 500 cubic centims. of water, form a liquid which gives good results even when a great portion of the manganese is separated by the electrolytic process. Concentrated solutions are by no means applicable; even the liquids which Nobili (Pogg. Ann. vol. x.) and Böttger (vol. 1.) used for the preparation of the coloured rings are too concentrated. A weakly-charged element is used along with the decomposition-apparatus described for hydrated peroxide of lead; as the liquid is colourless, the progress of the interference-bands during the operation can be observed through the spectroscope. The strength of the current is best directed so that the layer grows half a wave-length within from fifteen to thirty minutes; if the growth goes on too quickly, the thicker bright blue-black layer becomes brittle, and with a change of temperature, especially by a wash of cold water, easily cracks.

To determine the specific gravity, layers of more than 100 wave-lengths in thickness and of the absolute weight of about 0.5 grm. were prepared; from two concordant experiments the density at 13° C. was

s=2.542.

It

The body is not MnO2, but, like all bodies of this group, a hydrate which does not lose its water under the air-pump. is transparent to green and blue rays in quite thin layers only, of from 1 to 2 wave-lengths, so that minimum-bands can be observed in E and F; with greater thicknesses the same only appear in the yellow and red. The minimum in F, however, is so broad and faint that the wave-length cannot be determined from it by the simple spectroscope without the application of photometrical means. For the lines E, D, C, I have obtained according to the foregoing methods the values

n(E) = 1.944,

n(D) = 1.862,

n(C) = 1.801.

III. General Conclusions.

Besides the bodies already described, I also prepared a number of interference-layers in electrolytic and chemical ways, which, like those described, are distinguished by an unusually strong dispersion. The examination of these bodies has led to the conclusion that all bodies of strong dispersion have optical properties in common which appear of interest for the theory of light.

It is known from experience that dispersion and absorption are related to one another; and Cauchy, in his "Mémoire sur la dispersion de la Lumière," has given an equation in which this

116 On the Refractive Indices and the Dispersion of Opaque Bodies. relation is implicitly contained. The discussion of this equation, of which one side is an infinite series, presents some difficulties; it has been thought* sufficient to retain the first two terms of this series and neglect the rest. This would be permissible, as M. Christoffel has shown, if in every case the sphere of action were infinitely small in comparison with the wave-length. That the last assumption is not admissible, however, the discussion of the incomplete equation shows; for it yields the result that every spectrum is bounded at the violet end by a visible beam of definite refraction. This inference is a physical absurdity, since it presupposes the existence of bodies which, under any arbitrary angle of incidence, totally reflect a visible beam, or completely absorb it on the surface. The dispersion-formula derived from the imperfect series, even if correct for large wave-lengths, can offer no explanation as to what really occurs at that limit at the more refrangible end of the spectrum.

Whilst this limit with substances of weak dispersion would lie very far in the ultra-violet, it sometimes appeared in the green in the bodies which I investigated. With no single body of this group were even traces of interference observed in the violet. The reason of this phenomenon might be sought in a strong reflection of these rays at the surface, or in a strong absorption in the interior; it has been shown that the latter is the preponderating cause of the absence of interference-bands. Then they always vanish gradually with increasing thickness from the violet to the red end of the spectrum, and are very soon only present in the yellow and red. Hence the absorption increases with decreasing wave-lengths, and indeed continuously so for a certain position in the spectrum which is special to each substance, and so quickly that on the other side of it no ray can pass through a layer of the thickness of half a wave-length.

Hence in transmitted light sufficiently thick layers of bodies of preeminent dispersion always appear yellow-red or red. I have sought in vain for a substance of this kind which would be transparent with green, blue, or violet light.

To meet any objections to these matters of fact arising from the mention of apparent exceptions, I must make the following remarks.

Thin layers can be prepared in different ways which strongly absorb the light and are transparent to other than yellow or red light; such layers, however, like glass coated with soot, are not to be regarded as bodies, but as loosely connected apparatus of individual particles, and can only be quoted as exceptions if it be proved generally that they possess refractive and dispersive * Cauchy, "Mém. sur la Disp.," and Christoffel, Pogg. Ann. vol. cxvii. pp. 27-45.

properties. For example, let chlorine, bromine, iodine, sulphur-vapour, or sulphuretted hydrogen act on thin layers of silver; then layers of chloride, iodide, and sulphide of silver are obtained, which in comparison with the metals and the metallic oxides described are very transparent, and show in the spectroscope beautiful interference-bands. If, however, the intensity or duration of the action of those agents exceeds a certain limit, the structure of the layers is destroyed; the same are then to be regarded as aggregates of many particles (in several cases microscopic crystals), although they appear to the eye as coherent masses; they are more opaque than the metal itself, and show no trace of interference-bands in the spectroscope.

In reference to the chemical combination of the oxygen-compounds of the heavy metals prepared by electrolysis, the following result has been found:-Those compounds separated by the current at the positive pole are not, as has hitherto been commonly assumed*, peroxides, but definite hydrates of the same, which do not lose their moisture under the air-pump. I believe I am able to lay this down as a general proposition, as I have proved it for the most different metals, namely lead, manganese, cobalt, bismuth, and antimony. The oxides and suboxides separated at the negative pole are, on the contrary, always free from water, as must be inferred from the examination of the electrolytic suboxides of copper, bismuth, antimony, and oxide of iron, Berlin, October 1869.

XV. On the Determination whether the Corona is a Solar or Terrestrial Phenomenon. By GEORGE M. SEABROKE, Esq.† IT is my intention in this paper to attempt to show that, with

the existing state of our knowledge of the corona, the theory set forth by Mr. Lockyer, that the corona is a terrestrial phenomenon, is quite possible, rather than to show that other theories are wrong; and further to demonstrate how the question may be set at rest by observations on future eclipses. The points which present themselves are as follows:

1. What are the facts with respect to the spectra of the corona seen in past eclipses?

2. What spectra ought we to obtain from the corona on the terrestrial theory during totality?

3. Are the spectra obtained from the corona in past eclipses reconcilable with those we ought to get on the above hypothesis?

* Vergl. Wöhler, "Ueber das Verhalten einiger Metalle im el. Strom," Nachr. der Kgl. Ges. d. Wiss. u. der G. Univ. zu Gött. 1868, No. 8.

† From the Monthly Notices of the Royal Astronomical Society, June 10, 1870.

4. What spectrum ought we to get from the corona after totality? 5. What spectrum ought we to get before totality on the following side of the moon?

6. What difference will there be between the spectrum of the central portions of the corona and that of the distant parts during totality?

With regard to 1. During the Indian eclipse, Major Tennant writes:- Directly I saw the whole moon in the finder I set the cross-wires immediately outside its upper limb. By the time I got to the spectroscope the cloudy range seen in the photographs had vanished from the slit, and I saw a faint continuous spectrum. Thinking that want of light prevented my seeing the bright lines which I had fully expected to see in the lower strata of the corona, I opened the jaws of the slit and repeatedly adjusted by the finder, but without effect. What I saw was undoubtedly a continuous spectrum, and I saw no bright lines. There may have been dark lines, of course; but with so faint a spectrum and the jaws of the slit wide apart they might escape notice." With respect to the American eclipse, Professor Pickering, with an ordinary chemical spectroscope directed to the sun's place during totality, saw a continuous spectrum with two or three brright lines, one "near E" and a second "near C." Professor Young, while examining a part of the prominence at +146°, saw C, near D, a line at 1250+20, and another at 1350±20, and the 1474 K line very bright, but not equal to C and Dg; but he observed that the 1474 K line, unlike C and D, extended across the spectrum; and on moving the slit away from the prominence it persisted, while Dg disappeared. He also believes that the two faint lines between it and Dg behaved in like manner. On examining a prominence on the other side of the sun, he observed nine lines and a faint continuous spectrum without any traces of dark lines in it.

As to the second point, let us find what spectrum we ought to obtain from a corona at a point on the earth where the limbs

G

E

of the sun and moon are in line,—that is, where the eclipse is total exactly.

Let A be a point on the earth where the sun is eclipsed;

BC, limit of earth's atmosphere;

D, the moon ;

HE, photosphere of sun;

EF, the apparent corona.

Now, if the corona be terrestrial, the light producing it must be reflected or separated from the atmosphere within the triangle ABC.

Join BD and produce to G.

Then G is the most distant point from the limb on the sun's disk from which light is reflected to A by the atmosphere; and if the triangle EAF or angular extent of the corona from the sun is given, we can find EA G.

LEAG

The angles being small, ZEAF

=

GE

EF approximately.

GE:CB::ED: DC, therefore GE=CB

and

EF:CB:: EA: CA, therefore EF-CB;

EA

EF

CA

(2)

and ED=EA—AD; and AD being small in proportion to EA, ED may without great error be taken as equal to E A. Dividing (1) by (2),

Dist. of moon-height of atmosphere =240,000 miles,

then

LEAG=30'

100 240,000

=0"-75.