It is remarkable that the easily resolved cluster in Hercules has a spectrum precisely similar. The prismatic connection of this cluster with the nebula in Andromeda is confirmed by telescopic observation. Lord Rosse has discovered in this cluster dark streaks or lines similar to those which are seen in the nebula in Andromeda. In connection with these observations, it was of great interest to ascertain whether the broad classification afforded by the prism of the nebulæ and clusters would correspond with the indications of resolvability furnished by the telescope. Would it be found that all the unresolved nebulæ are gaseous, and that those which give a continuous spectrum are clusters of stars? Lord Oxmantown has examined all the observations of the sixty nebula and clusters in my list, which have been made with the great reflecting telescope erected by his father the Earl of Rosse. The results are given in this diagram: Clusters Resolved, or Resolved?.. Continuous Gaseous .10 Resolvable, or resolvable? Blue or green, no resolvability, no resolvability seen.............. Not observed by Lord Rosse 0 0 Considering the great difficulty of successful telescopic observation of these objects the correspondence between the results of prismatic and telescopic observation may be regarded as close and suggestive. Half of the nebula which give a continuous spectrum have been resolved, and about one-third more are probably resolvable; while of the gaseous nebulæ none have been certainly resolved, according to Lord Rosse. The inquiry now presents itself upon us, what superstructure of interpretation have we a right to raise upon the new facts with which the prism has furnished us? Is the existence of the gaseous nebulæ an evidence of the reality of that primordial nebulous matter required by the theories of Sir William Herschel and Laplace. Again, if we do not accept the view that these nebulæ are composed of portions of the original elementary matter out of which suns and planets have been elaborated, what is the cosmical rank and relation which we ought to assign to them? As aids to a future determination of these great questions I will refer in a few words to some other observations. COMETS. There are objects in the heavens which occasionally, and under some conditions, resemble closely some of the nebulæ. In some positions in their orbits some of the comets appear as round vaporous masses, and, except by their mo tion, cannot be distinguished from nebulæ. Does this occasional general resemblance indicate a similarity of nature? If such be the case, if the material of the comets is similar to that of the nebulæ, then the study of the wonderfel changes which comets undergo in the neighborhood of the sun may furnish useful information for a more correct interpretation of the structure and condition of the nebulæ. In 1864 Donati found that the spectrum of a comet visible in that year consisted of bright lines. Last January a small telescopic comet was visible. Its appearance in a large telescope is represented on the screen. It was a nearly circular, very faint vaporous mass. Nearly in the centre, a small and rather dim nucleus was seen. When this object was viewed in the spectroscope, two spectra were distinguished. A very faint continuous spectrum of the coma showing that it was visible by reflecting solar light. About the middle of this faint spectrum a bright point was seen. This bright point is the spectrum of the nucleus, and shows that its light is different from that of the coma. This short bright line indicates that the nucleus of this comet was self-luminous, and further, the position of this line of the spectrum suggests that the material of the comet was similar to the matter of which the gaseous nebula consist. MEASURES OF THE INTRINSIC BRIGHTNESS OF THE NEBULE. It appeared to me that some information of the nature of the nebulæ might be obtained from observations of another order. If physical changes of the magnitude necessary for the conversion of the gaseous bodies into suns are now in progress in the r bulæ, surely this process of development would be accompanied by marked changes in the intrinsic brightness of their light, and in their size. Now since the spectroscope shows these bodies to be continuous masses of gas, it is possible to obtain an approximate measure of their real brightness. It is known that as long as a distant object remains of sensible size, its brightness remains unaltered. By a new photometric method, I found the intrinsic intensity of the light of three of the gaseous nebulæ in terms of a sperm candle burning at the rate of 158 grains per hour: Nebula No. 4,628, 150g part of the intensity of the candle. Annular nebula, Lyra, part of the intensity of the candle. These numbers represent not the apparent brightness only, but the true brightness of these luminous masses, except so far as it may have been diminished by a possible power of extinction existing in cosmical space, and by the absorption of our atmosphere. It is obvious that similar observations, made at considerable intervals of time, may show whether the light of these objects is undergoing increase or diminution, or is subject to a periodic variation. If the dumb-bell nebula, the feeble light of which is not more than one twenty-thousandth part of that of a candle, be in accordance with popular theory a sun-germ, then it is scarcely possible to put in an intelligible form the enormous number of times by which its light must increase before this faint nebula, feebler now in its glimmering than a rushlight, can rival the dazzling splendor of our sun. MEASURES OF THE NEBULE. Some of the nebulæ are sufficiently defined in outline to admit of accurate measurement. By means of a series of micrometric observations, it may be possible to ascertain whether any considerable alteration in size takes place in nebulæ. METEORS. Mr. Alexander Herschel has recently succeeded in subjecting another order of the heavenly bodies to prismatic analysis. He has obtained the spectrum of a bright meteor, and also the spectra of some of the trains which meteors leave behind them. A remarkable result of his observations appears to be that sodium, in the state of luminous vapor, is present in the trains of most meteors. CONCLUSION. In conclusion, the new knowledge that has been gained from these observations with the prism may be summed up as follows: 1. All the brighter stars, at least, have a structure analogous to that of the sun. 2. The stars contain material elements common to the sun and earth. 3. The colors of the stars have their origin in the chemical constitution of the atmospheres which surround them. 4. The changes in brightness of some of the variable stars are attended with changes in the lines of absorption of their spectra. 5. The phenomena of the star in Corona appear to show that in this object, at least, great physical changes are in operation. 6. There exist in the heavens true nebula. These objects consist of luminous gas. 7. The material of comets is very similar to the matter of the gaseous nebula, and may be identical with it. 8. The bright points of the star-clusters may not be in all cases stars of the same order as the separate bright stars. It may be asked what cosmical theory of the origin and relations of the heav enly bodies do these new facts suggest? It would be easy to speculate, but it appears to me that it would not be philosophical to dogmatize at present on a subject of which we know so very little. Our views of the universe are undergoing important changes; let us wait for more facts, with minds unfettered by any dogmatic theory, and therefore free to receive the obvious teaching, whatever it may be, of new observations. Star differs from star in glory, each nebula and each cluster has its own special features; doubtless in wisdom, and for high and important purposes, the Creator has made them all. ON THE EXTERNAL APPEARANCE OF THE SUN'S DISK. TRANSLATED FROM THE GERMAN PERIODICAL "AUS DER NATUR," ETC., LEIPZIG, No. 11-1864. IN one of the last meetings of the Astronomical Society of England, some interesting remarks on the aspect presented by the exterior envelope of the sun were offered by M. Dawes. He first adverted to the great increase in the number of the observers who apply themselves to the study of that orb. since the number of large telescopes had itself so greatly increased and the dangers incident to the sight been correspondingly diminished. But the appearances in the sun, when carefully studied with these powerful instruments and under favorable atmospherical circumstances, differ so widely from those which had been observed with the imperfect instruments of former times, that it can occasion no surprise if some observers, unapprised of what has been already noticed, should take the objects which fall within the field of their telescope to be new discoveries. That this is sometimes the case, there is every reason to believe. Not this circumstance alone, but still more the new names invented for these supposed discoveries, tend certainly more to impede than to promote the advancement of science. It would seem desirable, therefore, that attention. should be directed to those appearances which have been long since observed and described, and that they should be collected and compared with the facts which have been more recently verified by the help of the improved telescope. To recognize the spotted appearance of the sun, no very great enlargement is necessary. M. Dawes has often observed it with a refractor of only 24 inches opening, and a magnifying power of 60. If the surface of the sun be observed with an instrument of from six to eight inches opening, it presents the appearance of being chiefly composed of luminous masses which are separated from one another by rows of small black points. The interval between these points is but feebly brought out, and is filled with a matter which shines less brightly than the general surface. Whatever the magnifying power employed, the division between the luminous masses seems never to be complete. These masses present almost all possible varieties of irregular forms. The rarest of all are those which Nasmyth has compared to willow leaves. They are long, slender, and pointed. This form is only observed in the immediate neighborhood of the larger spots, in their half-shade, and their shadows extending but a small distance from them—a peculiarity mentioned by Dawes as early as 1852, in his description of a new telescope, wherein he also states that the ander side of the half-shades appears thickly indented, that lustrous points seem to be directed to the centre of the spots, and that they present, on the whole, the ippearance of a band woven of straw whose inner ends have not been brought ato close conformity. Sir John Herschel has stated, in his Introduction to Astronomy, that the part of the sun's disk which is free from spots, shines with no uniform lustre; that he surface of the sun appears dotted with small black points or pores, which, when attentively observed, appear to be undergoing constant change, and that othing can be more fitly compared with this appearance than the slow subsidnce of the flakes of a chemical precipitation in a transparent liquid when looked t from above. M. Dawes has observed and confirms this phenomenon, though he throws some doubt upon the circumstance of a continual change in the state of these pores. He says that he has explored and considered the surface of the sun with the greatest care, reducing the diaphragm of his telescope to small openings of from 20 to 60" diameter, and availing himself of a greater degree of enlargement than is generally to be commanded; that he has frequently retained the same shining masses, with the included pores, under observation for not less than two hours, but has seldom at any time observed a change, even with a magnifying power of 4,600 fold. He adds that the disturbances which so often take place in the atmosphere are sufficient to produce the belief that a constant change is going on in the objects we observe, and that the eye, when confined to so narrow a field, soon grows weary, and the vision becomes embarrassed. To this comparative repose, however, there is one fact which forms an excep tion, for when our observation is directed to the immediate neighborhood of the spots, these are found to enlarge or diminish with great rapidity. It is chiefly under these circumstances that the lustrous masses assume the lengthened shape before spoken of; but these changes are the most vivid when these bright masses extend themselves by a rapid movement across the abyss, and thus form those dazzling bridges which often proceed from the principal spots. The point from which such a movement proceeds is frequently indicated by an accumula tion and bending of the greater axis of one of the lengthened masses in the direction of the movement. Here M. Dawes shares the opinion of Sir John Herschel respecting the appearance of chemical precipitation, and refers to it the cause of these phenomena. Wherever this appearance occurred, M. Dawes narrowed his observation to the edge of the spots, and embracing but a small field, examined with attention. the formation of the first part of the bridge. The bright masses presented the appearance of haums of straw, lying nearly all in the same direction and but few oblique to the line of the bridge, the sides of which seemed notched, on account of the unequal length of the parts of which it was formed. It may be remarked that these bridges are always constituted by bright stripes or lines which proceed from the outer envelope and are projected to the half-shadows, withoat mingling with the under and less brilliant strata. M. Dawes at least has never found it otherwise. The light of these stripes has always been of such intensity that the lines formed by the bridges, however narrow, rendered it impossible to distinguish with the eye the spot from the shadow. As regards investigations respecting the origin or cause of the sun-spots, M. Dawes advises observers to direct particular attention to the dark nuclens which presents itself in the shadow of the most symmetrical spots. For twelve years has he been made sensible of the inconvenience arising from the application of the same name to different objects and the failure to distinguish the shadow from the nucleus. This admonition has unfortunately been little heeded; in the description of the spots the shadow is constantly confounded with the true nucleus. M. Dawes the more insists upon this point, because his own observations have satisfied him that the presence or absence of the nucleus is competent to determine the origin of the spots, or at least to throw a strong light upon the question, and that the origin of the spots in which the nucleus occurs is alto gether different from that where no such phenomenon appears. To avoid all confusion it is of course indispensable to denominate each sereral thing by its true name. From the diligent observation of Dawes it results that the shadow under the half-shadow is not the sun's nucleus, and that this nucleus sometimes appears in the centre of the shadow. These two essentially different facts should never be lost sight of. |