when the light is feeble, the lines as they appeared in the still feebler light of the atmosphere near the sun's edge were carefully compared with the same lines in the spectrum of the umbra. The lines in the former case, though they appeared slightly stronger, were not so in a degree that could be accepted as an explanation of the more marked increase of strength which they presented in the spectrum of the umbra. There seemed, therefore, to be evidence of a peculiarity due to the light of the umbra itself.

In the spectrum of the umbra, which was sufficiently extended to show all the lines in Kirchhoff's maps, no lines were detected which were not also present in the spectrum of the sun's normal surface, nor were any lines observed to be wanting.

The increase of thickness did not take place in the same proportion for all the lines. The lines C and F, due to hydrogen, appeared to be increased but in a very small degree, not more so than would be due to the feebler intensity of the light.

There is a small group of lines a little less refrangible than b, at 1601 to 1609 of Kirchhoff's scale, and which in his map are marked as coincident with chromium, which were increased in a very marked degree. The lines D appeared in a small degree broader, as if by the addition of a faint and narrow nebulosity at both edges (see fig. 3. PI. V.). The group of lines at B was stronger, also the lines b and E and many lines found by Kirchhoff to be coincident with lines of iron. The absence of sensible increase in F was marked, in comparison with the greater strength of a line or lines, on the less refrangible side of F, at about 2066-2 to 2067·1 of Kirchhoff's scale. No bright lines were detected in the spectrum of the umbra.

It may be permitted to refer to some of the conditions of the solar surface by which the phenomena observed might be brought about*.

A cooler state of the heated vapours by which the dark lines of the solar spectrum are produced would diminish the radiation from the gas itself, and thus leave more completely uncompensated the absorption by the gas of the light from behind it. Such a cause would produce increased blackness of the lines, but would not account for more than a very slight apparent increase of breadth. The greater breadth of the lines may point to a condition of the solar vapours in which their power of absorption embraces, for each line, a greater range of wave-length. Such an alteration we know to occur in hydrogen as its tension increases. It may therefore be due to an increase of the density of the vapours existing within the umbra.

We do not know from how a great a depth below the layer of bright granules the light came that we have now under consideration. Probably it was emitted, for the most part, by that part of the sun which Mr. Dawes has named the cloudy stratum.

We have at present no certain knowledge of the true nature of a solar spot. Telescopic observation would seem to suggest that it consists essentially of the unveiling, by the withdrawal and dissipation of the layer of bright granules, of that part of the sun which is immediately beneath the granules, and of which we obtain some glimpses through the pores, which are always present, and are of different degrees of blackness.

The absence of bright lines from the spectrum of the umbra may show that no considerable part of the light which emanates from the umbra of a spot is due to luminous gas. This negative evidence, however, is probably

* [The absence of increase in the lines C and F may show that the absorption by hydrogen is not materially increased, or it may be caused by the bright lines of prominences İying over the umbra of the spot.-January 1869.]

not sufficient to support the conclusion that no part of the light of the umbra is from such a source. The luminous gas would emit light of the same refrangibility as some of the dark lines of the solar spectrum. If these existed above the same substance in a cooler state, the light might be absorbed, and the feebler emanations of the still luminous but cooler vapours might not do more than render less intense the dark gaps produced by the vapours on the stronger light of all refrangibilities which is also present. What may be the source of this light we do not know. It is not impossible that the dense and intensely heated gases which probably form the inner substance of the sun may in some cases emit lines so greatly expanded as to form, when numerous spectra are superposed, a sensibly continuous spectrum. Gases, when dense, appear to give a continuous spectrum, in addition to that consisting of bright lines.

§ V. OBSERVATIONS OF THE PLANETS.

Mars. In a paper presented to the Royal Astronomical Society in March 1867*, I gave the results of a further examination of the spectrum of Mars. In that paper I describe more fully than in my former papers the lines in the spectrum of this planet, which show the existence of an atmosphere similar to that of the earth, though probably not identical with it.

I give reasons which appear to show that the distinctive ruddy colour of the planet is not to be attributed to the absorptive properties of its atmosphere, but to some peculiarity which attaches to certain parts of its surface. Neptune.-I have several times observed the spectrum of Neptune, but failed to detect any very marked lines of absorption which might account for the blue colour of the planet. The faintness of its spectrum does not permit any great value to be attached to this negative result.

On Stellar Spectrometry. By Padre SECCHIT.

FRAUNHOFER Was the first to analyze with the prism the light of some of the stars. He discovered in them lines analogous to those which he had discovered in the solar spectrum. Donati, an Italian astronomer now at Florence, resumed these researches and extended their field. Several astronomers followed, and amongst them the distinguished Mr. Huggins, to whom we owe a description of the spectra of a great number of stars and the application of the principle of determining the substances contained in a star from the black lines of absorption which we see in its spectrum, as was proposed by Kirchhoff. Mr. Huggins also made the wonderful discovery of the gaseous state of the nebula.

The field opened by these discoveries was immense, and even before the date of Mr. Huggins's publications I tried to glean some ears in it. In the first stage of these studies the principal stars only were examined, the imperfection of my instruments not allowing the examination of all the heavenly bodies.

An optical combination which I had the good fortune to discover, enabled me to extend the researches to the whole of the visible stars, and even to several telescopic ones, which present perhaps the greatest mysteries of this kind.

*Monthly Notices of the Royal Astronomical Society, vol. xxvii. p. 178.
A communication ordered to be printed in extenso.

This optical combination consists of a single prism of that kind which is used for direct vision, combined with a cylindrical lens. This combination allows us to employ the full light of the stars, not diminished as in common spectroscopes by absorption, or by a slit and the several surfaces and thicknesses through which the light must pass. The image of the star in this system is formed in the focus as a luminous line of white colour if there is no prism; and with the prism the image is decomposed into a series of luminous lines arranged according to their refrangibilities, the interruptions due to the discontinuity of the light appearing as black lines.

In such a spectrum the relative position of the lines can be measured with a common screw-micrometer; and their absolute position can be determined by comparison with fundamental stars, whose lines, on account of their intensity, can be fixed in an absolute manner relatively to known substances by a common slit-spectroscope. The comparison and measurement are rendered more easy by an improvement introduced in the instrument, by means of which I can see the direct image of the star together with its spectrum. The superposition of this image on a spectral line in a part of the field of the telescope, marked by a wire, is susceptible of great nicety in measurement, and gives very accurate results.

This, in a few words, was the apparatus employed in my researches. This year I have made a considerable improvement by employing an eyepiece made with cylindrical lenses only; with these such an intensity of light is obtained that I have been able to observe the spectra of stars of the seventh and eighth magnitudes, which are of course quite invisible to the naked eye. Let us come now to the results. Many hundred stars of every magnitude to the sixth were passed in review. A catalogue of the chief of them has been made, and partly published. The work of the last year, yet unpublished, has been especially the examination of the red stars of smaller magnitudes, of which a particular research was instituted, but which was superseded after the reception of the catalogue of Prof. Schjelerup. All the objects contained in this catalogue (printed also in Chambers's Treatise on Astronomy) have been examined to the eighth magnitude, beyond which limit my instrument cannot give a good spectrum.

The principal results and conclusions at which I have arrived are these:1st. All the stars in relation to their spectrum can be divided into four groups, for each of which the type of spectrum is quite different.

The first type is represented by the stars Sirius, and Vega or a Lyræ, and by all the white stars, as a Aquila, Regulus, Castor, the large stars in the Great Bear, a excepted, &c. The spectra of all these stars consist of an almost uniform prismatic series of colours, interrupted only by four very strong black lines. Of these black lines the one in the red is coincident with the solar line C of Fraunhofer; another, in the blue, coincides with the line F; the other two are also in the sun's spectrum, but they have no prominent place. These lines all belong to hydrogen gas; and the coincidence of these four black lines with those of the gas has been, by careful experiments, already proved by Mr. Huggins, and also lately by myself. In a Lyra the coincidence is found to be perfectly accurate. Mr. Huggins, however, finds a little difference in the spectrum of Sirius, for which we may account in another way, as I will explain presently.

Stars of this first type are very numerous, and embrace almost one-half of the visible stars of the heavens. We observe, however, some difference in individual stars; so that in some the lines are broader, and in others narrower; this may be due to the thickness of the stratum which has been traversed by the luminous rays. The more vivid stars have other very fine

lines occasionally visible, but which are not characteristic of the type-form. In this type the red rays are very faint in proportion to the blue, violet, and green, so that the colour of the star tends to the blue hue, and occasionally to the green. Of this last kind is the group of the large constellation Orion and its neighbourhood. The second type is that of the yellow stars, as Capella, Pollux, Arcturus, Aldebaran, a Ursa Majoris, &c. These stars have a spectrum exactly like that of our sun-that is, distinguished by very fine and numerous lines. These stars give occasionally a continuous spectrum, when the state of the atmosphere is not good; but in general the lines may be distinguished very easily. A fuller description is unnecessary, since the spectrum of the sun is very well known. The only thing which deserves particular attention is that in this class occasionally the magnesium lines are very strong, so as to produce very strong bands, and the iron lines in the green are in some very distinct. These stars can be distinguished even without the prism by the difference of colour, a rich yellow, which contrasts strongly with that of the first type. Stars of this second type are very numerous, and embrace almost the other half of the stars.

The third and very remarkable type is that of orange or reddish stars. These have as a prototype the stars a Herculis, a Orionis, Antares, o Ceti, B Pegasi. The spectra of these stars show a row of columns at least eight in number, which are formed by strong luminous bands alternating with darker ones, so arranged as to represent apparently a series of round pillars, closely resembling a colonnade. a Herculis is exceedingly remarkable in this respect; the other stars are more or less clearly divided into pillars; but it is quite impossible to describe the beauty of the appearance which is visible in a telescope on a fine night.

All the pillars are generally resolved more or less completely in different stars into smaller and finer lines, very sharp and clear. I have carefully drawn, after actual measurements, the spectrum of a Orionis and a Herculis; and in my memoir those of Antares and Aldebaran are given. In these stars some of the divisions of the pillars correspond to some principal lines of Fraunhofer, as D and b; but others, although very near, do not coincide with them, as C and F. The presence of hydrogen, however, is certain, the lines C and F having been found in the principal of them.

The divisions of the pillars after many measurements have been found to agree perfectly in all these stars; so that this type is very constant and well marked. In my catalogue 25 of these most interesting objects are registered; and I do not imagine that I have exhausted the number.

A very interesting feature connects this type with the preceding one. Here I must remark that we have to distinguish between lines and bands of shadow. The lines are strips narrow and sharp, the bands are shaded; although perhaps each band may be composed of very small lines, the aspect with our instruments (as at present constructed) is that of a more or less continuous shade. This shade is analogous to that which is produced by the vapour of our atmosphere in the spectrum of the sun when it is near the horizon.

Now it is a very remarkable fact that these types seem to differ from one another not in the metallic lines, but in the nebulous bands. Thus, for instance, the spectrum of Arcturus and Aldebaran represent the same metallic lines as a Orionis, but this has bands in addition; the feature, however, is altogether so peculiar that a different type must be constituted. It is to be remarked also that all the pillars have their luminous sides toward the red,

very

sub

while the shadowed sides are towards the violet; this difference is
stantial, as we shall see presently.
The following is a list of the most remarkable stars of this type:-

(Their positions can be obtained from Chambers's Astronomy.')

The fourth type is not less remarkable. This is the result of a laborious research on the telescopic stars of a red colour. Some of these are very small; and none of them exceed the sixth magnitude. This is the reason why in my first memoir I limited the spectra to three types only, being engaged on larger stars only. The spectrum of this type consists of three large bands of light, which alternate with dark spaces so distributed as to have the most luminous side towards the violet.

A very fine prototype of this is seen in the small star of the Great Bear, of the position R.A.=12h 38m-5, Decl. 46° 15' N. But occasionally there are in the yellow and red numerous interruptions, which divide these large luminous spaces into smaller ones, as in the stars R.A.=22h 52m.5, Decl.=—25°51', and R.A.=6h 26.9, Decl.=38° 33'. A great part of the red stars of the catalogue of Lalande, and of that of M. Schjelerup, belong to this or the preceding type; of this last class I have found seventeen remarkable examples. The characteristic colour here also may be a guide in the research, since some of these are like drops of blood in the field of the telescope. It is to be noticed that the line of magnesium b falls almost exactly at the end of the second luminous band in the green; but the full aspect of the spectrum does not justify the presence of such metal, but rather of a gas like carbon, which has luminous bands corresponding almost to the dark ones of the star, but not exactly.

I do not attempt, however, to fix the nature of the substances, since I have not yet made a sufficient number of comparative measurements; but it seems to me that we are authorized in supposing these stars to be still in a different condition from others, perhaps partly in the gaseous state, or at least surrounded by a very large atmosphere different certainly from that of the others. The following is a Catalogue of these Stars of the fourth type.