prisingly well from night to night, while the former exhibit discordances of considerable amount. This it is difficult to account for, except on the supposition that the snowy cap before referred to may have had some influence in distracting the eye from the real borders of the images in making the contacts. Still, on the whole, the measures all agree in establishing a measurable ellipticity, and Mr. Main intended to continue them at every opportunity during the present opposition, with the utmost care and caution.

On some Peculiar Features in the Structure of the Sun's Surface.

By J. NASMYTH.

The author gave a short sketch of the character of the sun's surface as at present known. He described the spots as gaps or holes, more or less extensive, in the luminous surface or photosphere of the sun. These exposed the totally dark nucleus of the sun; over this appears the mist surface-a thin, gauze-like veil spread over it. Then came the penumbral stratum, and, over all, the luminous stratum, which he had discovered was composed of a multitude of very elongated, lenticular-shaped, or, to use a familiar illustration, willow-leaf-shaped masses, crowded over the photosphere, and crossing one another in every possible direction. The author had prepared and exhibited a diagram, pasting such elongated slips of white paper over a sheet of black card, crossing one another in every possible direction in such multitudes as to hide the dark nucleus everywhere, except at the spots. These elongated lens-shaped objects he found to be in constant motion relatively to one another; they sometimes approached, sometimes receded; and sometimes they assumed a new angular position, by one end either maintaining a fixed distance or approaching its neighbour, while at the other end they retired from each other. These objects, some of which were as large in superficial area as all Europe, and some even as the surface of the whole earth, were found to shoot in thin streams across the spots, bridging them over in well-defined streams or comparative lines, as exhibited on the diagram; sometimes by crowding in on the edges of the spot they closed it in, and frequently, at length, thus obliterated it. These objects were of various dimensions, but in length they generally were from 90 to 100 times as long as their breadth at the middle or widest part.

Observations on Three of the Minor Planets in 1860.
By NORMAN POGSON. Communicated by Dr. LEE, F.R.S.

Observations of Minor Planets made at Hartwell in 1860.

The first observation of Eunomia was made with the parallel wire micrometer, and power 110; all others with the ring micrometer, and power 84 of the Hartwell Equatorial. The comparison stars employed were as in the annexed list :

The following magnitudes have been carefully estimated; generally, by comparison with apparently similar objects in the nearest variable star-map then in course of construction:

The preceding observations of minor planets were the last made by Mr. Pogson before leaving England for Madras in January 1861; it was his intention to reduce them speedily, and to send them to me from Malta or Alexandria; but, as anticipated, the inconveniences of a sea-voyage prevented him from fulfilling his design, and the pressure of official duties in his new position has not permitted him to attend to his former unfinished pursuits until recently.

On the Excentricity of the Earth, and the Method of finding the Coordinates of its Centre of Gravity. By W. OGILBY, F.G.S.

On the probable Origin of the Heliocentric Theory. By J. SCHVARCZ. The author traced the origin of the Copernican system to Pythagoras, through Aristarchus the Samian and Archimedes of Syracuse.

On Autographs of the Sun. By the Rev. Professor SELWYN.

The author showed several "autographs of the sun," taken with his "heliautograph" by Mr. Titterton, photographer, Ely, which consists of a camera and instantaneous slide by Dallmeyer, attached to a refractor of 24 inches aperture by Dollond; the principle being the same as that of the instrument made, at the suggestion of Sir J. Herschel, for the Kew Observatory. The autographs are of July 25, 26, 28, 29, 31; August 1, 2, and August 4, 10.15 A.M. and 11.30 A.M. (a series of bright days coincident with a large group of spots); August 19, 20, 23 and 25, where the same group reappears, much diminished; September 19, 23, 26, 30, Oct. 1, in which is seen a group of 118,000 miles in length. On the 23rd three autographs were taken, two of them with the edge of the sun in the centre of the photographic plate, showing that the diminution of light towards the edges of the disk is a real phenomenon, and not wholly due to the camera. In the two of the 4th of August, where the great spot (20,000 miles in diameter) appears on the edge, a very distinct notch is seen, and the sun appears to give strong evidence that the spots are cavities; but eye observations and measurements by the Rev. F. Howlett, and others, tend to show that this evidence is not conclusive, for there was still a remaining portion of photosphere between the spot and the edge. The phenomena shown in these autographs appear to confirm the views of Sir J. Herschel, that the two parallel regions of the sun where the spots appear, are like the tro1862. 2

pical regions of the earth where tornadoes and cyclones occur, and those of Wilson in the last century. The faculæ are clearly shown, and seem to prove that the tropical regions of the sun are highly agitated, and that immense waves of luminous matter are thrown up, between which appear the dark cavities of the spots, whose sloping sides form the penumbræ, as explained by Wilson and others. Other analogies between solar spots and earthly storms were pointed out, and reference was made to the glimpses of the structure of the sun exhibited by Mr. Nasmyth as confirming the above views.

On the Hindu Method of Calculating Eclipses. By W. SPOTTISWOODE, F.R.S. The astronomy of the Hindus is contained in a series of works known by the general name of "Siddhanta." These have been composed at different times over a period of 2000 years. In them are some incidental allusions to the configurations of the heavenly bodies, by means of which Baily, Davis, and others have atte npted to calculate the dates of some of the works. There were two points to which the author drew particular attention, viz. the process of correction whereby the true longitudes were deduced from the mean, and the precession of the equinoxes. It had been noticed that the apsides, or points of slowest movement, and the positions of conjunction with the sun had proper motions. These were attributed to influences residing in the apsides and conjunctio's respectively, and corrections due to each were accordingly devised. The undisturbed orbit was considered a circle, with the earth (E) in the centre, and upon it the centre of a smaller circle or epicycle moved with a uniform angular velocity equal, but opposite in direction, to that of the undisturbed planet; so that M being the centre, ard m any given point on the epicycle, Mm always remained parallel to itself. If, then, at the apse or conjunction (according as the correction of one or the other was being calculated) Mm was in a straight line with EM, the true position of the planet was conceived to be at the point where Em cut the undisturbed orbit. The radius, moreover, of the epicycle was variable, and its magnitudes at the odd and even quadrants being determined so as to satisfy observation, its intermediate variation was considered proportional to the sine of the mean anomaly. The precession of the equinoxes is an important element in Hindû astronomy, not only as a question of scientific accuracy, but also as marking an epoch in the history of discovery. It is an ascertained fact that their earlier writers, among the foremost of whom Brahmagupta may be mentioned, took no account of it whatever. The statement in the Surya Siddhanta, when divested of its obscure terminology, seems to amount to this, that the sidereal circle shifts on the zodiac with an oscillating motion, whose period is 7200 years, and whose maximum range is 27°. This gives an annual rate of 54".

On some Improved Celestial Planispheres. By C. J. VILLA.

LIGHT AND HEAT.

On the Means of following the Small Divisions of the Scale regulating the Distances and Enlargement in the Solar Camera. By A. CLAUDET, F.R.S. The author, in a former paper, had proposed a new method for measuring both the distances of the negative and screen for any degree of enlargement of the image, by means of a scale or unity divided into 100 parts, and smaller fractions if possible. This scale being fixed on the table of the optical apparatus, an index connected with the frame holding the negative was brought exactly on any division of the scale which was indicating the proportion and distance of the image. This arrangement would be very complete and satisfactory if the scale were always long enough to be marked with divisions sufficiently conspicuous; but the shorter the focus of the object-glass, the smaller the divisions of the scale must be. In order to meet this difficulty, he has adopted the following plan:-He traces on the table an equilateral

triangle, the base of which is the exact length of the scale. Taking 8 inches, for example, as that length, the three sides of the triangle will be 8 inches. Now, it is possible to enlarge the base three, four, five, or any number of times, by extending the sides of the triangle in the same ratio; so that if it be desirable to enlarge the scale four times, a triangle is formed having its base four times longer, viz. equal to 32 inches; and dividing this new base into 100 parts, it is evident that each division will be four times larger than it could have been on the original base. Now, describing an arc, the chord of which is the base of the triangle, and attaching to the summit a thin metallic wire, the other end of which can slide on the arc, it is evident that each division of the magnified scale which may be covered by the wire will correspond exactly with an equal division of the original scale, so that, after having brought the metallic wire on the division of the increased scale indicating the size of the required image, and the wire being fixed on the index, it will be brought exactly on any division of the unity of measure, however small it may be, The author has described another plan to obtain the same result, and, perhaps, more effectively it consists in fixing the negative on a rack exactly the length of the scale, which, acting on a pinion adapted to a sufficiently large wheel containing the requisite divisions, will produce an entire revolution of the wheel; and an index being fixed on the table, will indicate on the wheel the exact amount of the course effected by the negative on the scale; and by turning the wheel to the division required, this will bring the negative with the greatest accuracy to the distance corresponding with the division. This system of focusing all camera-lenses might be very advantageous in photographic operations, and would be less subject to errors than the usual way of focusing on the ground glass.

Relation entre les Phénomènes de la Polarisation Rotatoire, et les Formes Hémiedres ou Hemimorphes des Cristaux à un ou à deux Axes Optiques, Par A. DES CLOIZEAUX.

Tout le monde sait que la découverte de la polarisation de la lumière a rendu possible l'institution de nombreuses recherches, inabordables à tout autre mode d'observation, sur la constitution moléculaire des corps solides et liquides. Je n'entreprendrai pas ici de passer en revue les faits intéressants et les lois remarquables dont on doit la connaissance aux travaux des Malus, des Fresnel, des Herschel, des Arago, des Brewster, des Biot, &c. Je m'occuperai seulement de la polarisation rotatoire et des relations que ce phénomène peut avoir avec la structure physique des corps cristallisés. Depuis que la science a été dotée des microscopes polarisants d'Amici et de Nörrenberg, on a pu étendre les observations optiques à un grand nombre de substances trop peu transparentes ou de trop petites dimensions pour se prêter à l'emploi des instruments généralement usités jusque dans ces dernières années. Le quartz est resté pendant très longtemps le seul corps solide dans lequel on eut constaté l'existence du pouvoir rotatoire, et Sir John Herschel a le premier fait remarquer qu'il paraissait y avoir une relation constante entre le sens de la rotation des cristaux et le sens suivant lequel s'enroule la spirale formée par plusieurs des faces connues sous les noms de faces plagièdres et par la face rhombe, lorsque l'axe principal des cristaux est placé verticalement devant l'observateur. Ce rapprochement à conduit à regarder le phénomène de la polarisation rotatoire comme dû à un arrangement particulier des molécules physiques qui se manifesterait quelquefois par des formes cristallines présentant l'hémiédrie dite plagièdre ou tournante. On sait que le caractère de cette hémiédrie est la non-superposition des solides symétriques résultant de la réunion des faces plagièdres situées à droite et à gauche d'une même face prismatique du quartz. L'observation prouve d'ailleurs qu'elle peut s'allier avec l'hémiédrie qui fournit pour la face rhombe deux solides inverses mais superposables. Il est en effet probable que c'est une structure de ce genre qui donne aux cristaux dextrogyres et aux cristaux lévogyres la propriété d'imprimer à la lumière polarisée des modifications de sens contraire; car on n'a jamais observé de phénomènes rotatoires dans les cristaux d'apatite, de Schéelite, d'érythroglucine, &c., sur lesquels on ne connaît jusqu'à présent que des formes hémièdres superposables. Malheureusement la dissymétrie intérieure n'est pas toujours accusée par des signes extérieurs, et l'observation seule indique si un corps cristallisé possède ou ne possède

a

pas la polarisation rotatoire. Ainsi, un grand nombre de cristaux de quartz ne portent aucune face plagièdre; le chlorate de soude, dans les cristaux duquel M. Marbach a découvert le pouvoir rotatoire, s'obtient tantôt en cubes parfaits, tantôt en tétraèdres simples ou en tétraèdres modifiés par les faces d'un dodécaèdre pentagonal qui occupent relativement à celles du tétraèdre deux positions inverses l'une de l'autre en rapport avec le sens de la rotation; le cinabre rhomboédrique et le sulfate de strychnine quadratique, qui, d'après mes observations, impriment aussi au plan de polarisation une déviation égale, pour le premier à 16 fois et pour le second à la moitié de celle que produit le quartz, n'ont offert jusqu'ici aucune trace d'hémiédrie; cependant j'ai trouvé dans le cinabre des cristaux dextrogyres, des cristaux lévogyres, et des cristaux complexes où l'emploi de la lumière polarisée convergente manifeste les spirales d'Airy absolument comme dans le quartz. La cause qui donne naissance à la polarisation rotatoire dans les cristaux paraît donc indépendante de celle qui produit les formes hémiédriques; seulement, comme l'a fait voir M. Marbach, la production de ces formes peut être favorisée artificiellement en faisant varier les conditions dans lesquelles s'opère la cristallisation. Il est donc probable que les cristaux de quartz à faces plagièdres n'ont pas pris naissance dans les mêmes circonstances que ceux où les faces plagièdres manquent; tous les cristaux de cinabre connus jusqu'à ce jour ont dû au contraire se former sous l'influence de phénomènes géologiques semblables.

Depuis que M. Biot a découvert la déviation imprimée au plan de polarisation par certains liquides et certaines dissolutions, on s'est souvent demandé si les dissolutions actives susceptibles de cristalliser produisaient nécessairement des cristaux doués du pouvoir rotatoire. La plus grande partie des substances actives en dissolution cristallisant sous des formes qui possèdent deux axes optiques, la question est longtemps restée sans réponse expérimentale. Mais les travaux de M. Marbach et les miens, en révélant l'existence des trois seuls cas réalisables dans les cristaux dépourvus de la double réfraction ou dans les cristaux à un seul axe optique, semblent prouver que les deux genres de phénomènes sont indépendants l'un de l'autre. En effet, 1°, le chlorate de soude, inactif en dissolution dans l'eau, jouit du pouvoir rotatoire lorsqu'il est en cristaux; le quartz fondu ou à l'état de silice soluble et le quartz cristallisé présentent les mêmes différences.

2°. Le sulfate de strychnine quadratique à 13 équivalents d'eau, en dissolution comme en cristaux, dévie à gauche le plan de polarisation, seulement le pouvoir rotatoire des cristaux est environ 30 fois plus grand que celui de la dissolution.

3°. Le camphre ordinaire des laurinées, actif en dissolution et à l'état fondu, donne par sublimation des cristaux appartenant au système hexagonal, dans lesquels on ne peut constater aucune déviation du plan de polarisation, même sous une épaisseur de plusieurs millimètres.

Les cristaux à deux axes optiques, dont la dissolution possède le pouvoir rotatoire, sont assez nombreux; on a donc pu les soumettre des expériences variées. D'après les recherches de M. Pasteur, l'existence du pouvoir rotatoire dans une dissolution serait le plus souvent (à l'exception des sulfamylates) accompagnée par l'hémiédrie non superposable ou l'hémimorphie d'une ou de deux des formes simples que présentent les cristaux dissous. Cette hémiédrie se montre d'ailleurs quelquefois sur les cristaux formés naturellement au sein d'une dissolution dans l'eau pure, d'autres fois elle doit être provoquée, soit en faisant varier la nature du dissolvant, soit en blessant les cristaux et les replaçant dans leur eau-mère*. S'il existe, comme pour l'acide tartrique, les tartrates et quelques autres substances d'origine organique, deux dissolutions, l'une lévogyre et l'autre dextrogyre, les formes hémièdres ou hemimorphes correspondantes produisent ordinairement (le sel de seignette potassique paraît seul faire exception) deux solides symétriques mais non superposables. La réciproque n'est pas vraie dans tous les cas, puisque le sulfate de magnésie et le formiate de strontiane, dont les cristaux offrent l'hémiédrie non superposable, fournissent des dissolutions inactives. Les causes qui produisent les formes cristallines hémièdres paraissent donc agir d'une manière plus générale que celle à laquelle est dû le pouvoir rotatoire moléculaire.

* Ann. de Chimie et de Physique, tom. xxxviii. et xlix,