globe there are many extended regions which have no points so accurately determined.

The year 1911 saw published the tables of the moon based upon the theory of Delaunay and forming volume 7 of the Annales du Bureau des Longitudes. An introduction of 112 pages, where not a line is superfluous, allows us to form an idea of the mangitude of the task. Delaunay, Tisserand, and R. Radau have successively given to the task the last years of their lives and were assisted by Schulhof. An analogous undertaking was carried out in America by Prof. E. Brown; the French astronomers by finishing first have honorably maintained the tradition established by Laplace and Le Verrier. All the errors, although very small, known in the works of Hansen and Delaunay, have been corrected. But it was impossible to break away from all empiricism and there remains an inequality of long period (273 years) discovered by Newcomb. Newcomb gave up trying to find an explanation. More optimistic, R. Radau believes the cause can be found in cosmic dust and the infra-mercurial planets.

A new determination of the parallax of the moon, due to the collaboration of the Cape and the Greenwich Observatories has been carried out after six years of work. The value generally used was confirmed.

III.

The result, at first sight rather small, of a considerable effort, is not to be understood as minimizing the desire often expressed of substituting for the classic method of charting the sky more rapid and more accurate methods. What is aimed at everywhere is the suppression of the measures of moderately large angles by the readings of divided circles. More confidence is placed in measures of time intervals and the extension of such a method is to be expected.

W. E. Cooke proposed, for the determination of right ascensions, not to use a meridian circle but rather a telescope whose optic axis when rotated about a vertical axis intersects in the heavens a small circle parallel to the horizon. We could by such means divide the celestial equator, or any parallel circle with an accuracy comparable with the precision of our best clocks. The declination of the stars would be determined from the times when they reached a determined altitude. The axis of the telescope must therefore be maintained at a constant altitude. At various times the realization of this condition has been attempted with telescopes on floating mounts. Cooke places faith in a vertical axis and level.

Among the disadvantages of the meridian instrument is the necessity that each celestial point has to be separately determined and that great intervals can not be measured as accurately as small ones.

1 Session of the Royal Astronomical Society, May 12, 1911.

That photography can deal with small intervals with rapidity and great accuracy has been repeatedly shown. But it is desired to free the photographic plate from any dependency upon the meridian circle. H. H. Turner1 has devised a very complete scheme. He proposes to gather, on the same plate, images of very distant portions of the sky, and believes he can register with the necessary precision the beginning of each exposure. The plan of Turner includes the use of two photographic telescopes mounted at right angles to each other in the equator and adjusted with a prismatic mirror. The project has received the approbation of the Astronomical and Astrophysical Society of America."

But these methods have not received such emphatic approval everywhere. Before the Royal Astronomical Society Sir David Gill, Sir William Christie, and A. E. Conrady showed numerous reasons for fearing errors in the use of the new methods. According to them, the status of the meridian circle in fundamental astronomy is not yet in any way undermined.

In the past rival processes, even when recommended by illustrious names, have not realized the hopes of their promotors. Such was the case with the zenith telescope, or the alt-azimuth as introduced at Greenwich by Airy and the floating telescope in the hands of Chandler, Sampson, and Bryan Cookson. But it is true that photography introduces a new element into the problem and the experiments now in progress at Oxford deserve attention.

The resources of the photographic method will be yet greater when it is possible to utilize a greater field upon a single plate without the deformation of the images near the edges. Theoretically, curved plates could be employed which would comport better with the focal surfaces of the objectives. Such an attempt was made some 20 years ago at the beginning of the Celestial Chart project. It was not continued in use because such curved plates were not adapted to the micrometrical measuring machines. The difficulty has been overcome as the result of recent experiments at the Harvard College Observatory. The sensitive plate serves as the cover of a metallic box, from which the air may be removed. The atmospheric pressure upon the plate produces the desired curvature. When the air has been reintroduced, after the exposure, the plate losses its curvature and is developed and measured without difficulty.

A conference was held in Paris in October, 1911, by the representatives of all the great nations which publish official ephemerides (France, Germany, the United Kingdom, the United States of America). The resolutions unanimously adopted after very amicable discussions will introduce important economies in efforts, which up to

Monthly Notices, vol. 71, pp. 422, 427.

The Observatory, vol. 34, p. 233.

the present time have been duplicated. The positions of the planets will be determined only by two independent systems of tables. Greenwich time will be universally used for the ephemerides and unity will be observed for the more important constants (solar parallax, precession, nutation, aberration). The campaign actively pushed during recent years for carrying the aberration constant from 20.47" up to 20.53" can be considered as having failed. The latter figure is irreconcilable with the solar parallax obtained from the general discussion of the Eros observations.

THE SPIRAL NEBULE. 1

By P. PUISEUX,

Member of the Academy of Sciences, Professor at the Sorbonne, Astronomer at the Paris Observatory.

The people whom we claim as our direct intellectual ancestors wished to find nothing in the sky but spherical forms and circular movements. The Greeks, lovers of an exact geometry, the Latins, enamored of order and logic, took pleasure in simple combinations. They would not willingly admit into the celestial throng clouds of indefinite and complicated form. Such indefinite forms must belong to the sublunar world. Comets, with their hairy aspect, passed as meteors, taking their birth and vanishing within our atmosphere. In the Milky Way some saw an accidental derogation of the universal order or the trace of an imperfect joining of the two halves of the celestial sphere. Others guessed it to be a mass of numberless stars, too small and too distant to be separately seen. There was no need, indeed no possibilty, of searching further. To those whom the idea of something beyond troubled, the existence of an empyrean was conceded, a luminous region situated beyond the stars, to which only those had access whose souls had become freed from the bonds of flesh.

But astronomy, no more than the other physical sciences, has kept within the bonds with which she was fettered in the name of philosophy. No sooner was the telescope invented than several observers used it to explore the sky. Then, as had been foreseen, the Milky Way was resolved almost entirely into separate luminous points. But, it is true, there were found a few refractory places, where the diffused whiteness persisted in filling the field of the telescope. Even outside the limits of the Milky Way, several such masses, more or less perceptible to the naked eye, refused to be decomposed. Simon Marius, in 1612, noted the great nebula of Andromeda, which suggested to him the comparison, somewhat trivial yet suggestive, of the flame of a candle seen through horn. This pale glow, watched for many years, seems to rest absolutely

1 Address delivered before the Société des Amis de l'Universté, Feb. 1, 1912. sion from Revue Scientifique, Paris, Apr. 6, 1912, pp. 417-422.

Translated by permis

unchanged when compared with the adjacent stars. It is therefore neither a planet nor a meteor. It belongs to our sun no more than to the earth. Accordingly, if we admit the Copernican theory, we must attribute to this nebula colossal dimensions, far exceeding the distance which separates the earth from the sun.

Nor is the nebula of Andromeda an isolated case. Christian Huygens made a drawing in 1656 of a nebula in the constellation of Orion, a more brilliant and more extended object the outlines of which he found very difficult to trace. On one side only was it sharply defined against the adjacent sky. Elsewhere it faded into indistinguishable nebulosity. Does it not seem, mused Huygens, as if here we are looking upon a new world, perhaps upon the legendary empyrean? This feeble veil scarcely alters the aspect of the stars which shine through it or are projected upon it.

This somewhat summary sketch of Huygens was only vaguely confirmed by those of Picard and of Legentil who came several years later. The only common trait, indeed, was the dark gulf which hollows out the central part. No part of it seemed sufficiently definite for the detection of possible changes.

During the eighteenth century the number of known nebulæ increased slowly. Several, upon closer examination, proved to be clusters of small stars. Those whose aspect remained flocculent, despite all efforts to resolve them, often deceived the comet seekers, who, after verification, saw their cherished hope of making the discovery of a new planetoid disappear. Messier, more than once thus caught, undertook to remove this cause of trouble and in 1784 published a catalogue of these objects, containing nearly all the nebulæ easily seen above the horizon at the latitude of Paris.

At the same time a great advance was made in England in the means of observation. The musician, W. Herschel, succeeded during his leisure hours in figuring and mounting telescope mirrors much greater and much more perfect than had ever been made before. In the field of these instruments nebulæ appeared in an absolutely unexpected profusion. Thus there arose a new branch of astronomy to be developed. Herschel set to work, aided by his sister Caroline, and with remarkable perseverence, at the same time pursuing other researches, catalogued, from 1786 to 1802, some 2,500 nebulæ. Many of these, upon closer examination, were resolved into stars. W. Herschel was led to believe that all could be so resolved and that any one of these scarcely visible flocculent specks would, to an observer properly situated, appear like a stellar universe as rich as that which surrounds us and which is evident to our eyes through the milky way.

The work of W. Herschel was completed for the Southern Hemisphere by his son John, who transported in 1834 to the Cape of Good Hope one of the best telescopes constructed at Slough by his father.