that the classification is carried decidedly too far, and loses clearness. Tabular statements, arranged by types, compare the three catalogues mentioned, and complete the memoir. As a whole this little volume affords an admirable illustration of what valuable work can be done with comparatively small, but finely adapted and adjusted, instruments, in (it should be added) the hands of experts. Those portions of the work, in part referred to above, for which the dispersion was inadequate, can be fully investigated with the splendid equipment now happily available for the future stellar spectroscopic work of the Potsdam. Observatory. E. B. F.

Comparative Photographic Spectra of Stars to the 31⁄2 Magnitude. By FRANK MCCLEAN. Phil. Trans., A. 191, 127–138, 1898. 17 plates.

Spectra of Southern Stars. By FRANK MCCLEAN. Pp. 16; 12 plates. London: Edward Stanford, 1898.

THE first of these two works is a collection of reproductions of 160 photographs of spectra of northern stars, accompanied by a brief discussion of their classification and distribution. No description is given of the instrument with which the plates were obtained, but from an earlier note by the author in Monthly Notices we may infer that an objective-prism of 12 inches aperture and 20° angle, mounted on a refractor of equal aperture and 11 feet 3 inches focal length, was employed.

As the ordinary data given with spectral plates, such as the date and duration of exposure, are also omitted, the intention of the work would seem to be pictorial and qualitative rather than metrical and quantitative. Regarded in this way, it is of decided value, being the only memoir to which one may turn for a photograph of the spectrum of each of the brighter stars. The plates are accompanied by representations of the spectra of several of the elements, for purposes of comparison and identification, and an approximate scale of wavelengths is given.

The author divides the celestial sphere into eight zones of equal area referred to the galactic poles, and briefly examines the distribution of the different types of spectra in these zones. He subdivides Secchi's first type into three "divisions," and makes divisions IV, V and VI

correspond to Secchi's types II, III and IV. A tabular statement is given of the arrangement of the stars on the plates which follow, together with the classification of each star according to Secchi, Vogel, Pickering, and Lockyer.

In an examination of the distribution of stellar spectra the arrangement of the successive spectra according to location in the galactic areas, and then according to type, is no doubt necessary, but for convenience of reference, an arrangement by types alone would be more convenient.

The plates themselves, covering the region from about A 3850 to A 4900, give evidence of the great efficiency of the objective-prism, although it is probable that the reproductions hardly do justice to the originals. There is clearly much room for improvement in guiding the telescope during the exposures, as the star lines are greatly inclined or curved on some of the spectra, thus requiring considerable ingenuity in fitting them together, one below the other, so that the scale of wavelengths could apply to all the spectra on a sheet. The plates in the second memoir, which is an extension of the first to the southern skies, show a very great improvement in this regard. The spectral lines are here always nearly vertical, and the appearance is much improved.

The second series of plates was made by the author during a visit to the Cape between May and October 1897, the objective-prism used in England for the first series being attached to the astrographic telescope of the Cape Observatory. The examination of the distribution of the different types is made in the same manner as in the first series, but the classification assigned by the other observers is necessarily omitted for these southern stars.

The chief interest in this memoir, aside from the general value of an accessible reproduction of the spectra of all the brighter stars, lies in the author's discovery of the presence of oxygen in the spectrum of B Crucis. The wave-lengths of the lines in the spectrum were measured and reduced by a cubical interpolation formula, and a table gives a comparison of these with the lines of helium, hydrogen and oxygen. Any lingering doubt as to the certainty of the coincidences with the oxygen spectrum is dispelled by the careful measurements made by Dr. Gill on spectra obtained with a large spectrograph and published in the last number of this JOURNAL.

Another table gives a list of the lines visible in the spectrum of y Argus, over fifty in number. The author identifies four of these

with lines of the series discovered by Pickering in the spectrum of Puppis. As two of these coincide with helium lines, which are shown by the author to be abundant in the spectrum, this identification perhaps required confirmation by measures on a plate taken with a spectrograph and having a suitable comparison spectrum.

E. B. F.

Annales de l'Observatoire d'Astronomie Physique de Paris. Tome premier. Paris, Gauthier-Villars et Fils. 4to pp. 122, plates XXI.

THE establishment of a national astrophysical observatory by the French government was a natural sequel to the able work of M. Janssen at the total solar eclipse of 1868. The discovery in that year of a method by which the spectrum of the chromosphere could be observed in full sunlight created the greatest enthusiasm, and the spectroscopic study of the Sun was undertaken in many European and American observatories. In Italy, thanks to the labors of Secchi, Tacchini, and Lorenzoni, there was established the Società degli Spettroscopisti Italiani, which gave every encouragement to astrophysical research. In England, Huggins made important contributions to our knowledge of the Sun, and did much to develop a method the potentiality of which he had long before foreseen. Lockyer, whose successful attempts to apply the spectroscopic method are also well known, sought in the study of solar phenomena data to support his hypothesis of the dissociation of the elements. In the United States Young entered upon the well-planned and well-executed series of observations which has added so much to our knowledge of the spectrum of the chromosphere, and Langley undertook his important solar investigations at the Allegheny Observatory. In Germany, Zöllner and Vogel recorded a great variety of interesting phenomena, and contributed to the improvement of the solar spectroscope. In France, however, as M. Janssen tells us in the historical sketch which introduces the present volume, the instrumental equipment required for solar research was lacking. A government project to establish a new observatory for M. Janssen might have been carried into immediate effect had not the Franco-Prussian War suddenly directed the attention of every Frenchman to the problem of the national defense. The outcome of the war, and the conditions which it entailed, were unfavorable to the development of the plan to establish

a national astrophysical observatory. Nevertheless the project was not forgotten. In July 1874 it was discussed in the National Assembly, and a few weeks later the Academy of Sciences was requested to report as to the advisability of founding an institution exclusively devoted to astrophysical research. The report of the special committee of the Academy, which was presented by M. Faye, may be found in the present volume of the Annales. After describing the first step in astronomical physics, which is credited to Arago, the report outlines the rise and progress of spectroscopy, refers to the discovery of the gaseous nature of the prominences by Janssen and Rayet at the eclipse of 1868, dwells on the nature and importance of astrophysical research, and concludes with a strong recommendation for the establishment of an astrophysical observatory in the immediate vicinity of Paris.

As a result of the recommendations of this report the government voted an annual grant of fifty thousand francs, which was devoted to the establishment and maintenance of a small observatory provisionally erected on the boulevard Ornano. The park surrounding the old chateau on the terrace at Meudon was selected as the permanent site, and preparations were made to occupy it as soon as it had been evacuated by the French troops, who at that time used the building for barracks. The sum necessary to reconstruct the chateau for observatory purposes and to purchase instruments, which amounted to 1,035,ooo francs, was appropriated by the Senate in three equal parts in the years 1879, 1880, and 1881.

The

The

M. Janssen's historical sketch is followed by a description of the great dome and the principal instruments of the Observatory. dome has a diameter of 18.5 m, and can be moved by electricity. observing slit is provided with an adjustable canvas screen to protect the telescope from the wind. Observations are made from a platform, attached to the dome opposite the slit, which can be raised and lowered by means of an electric motor. The telescope, with visual and photographic objectives by the Henry Brothers, of 0.83 m and 0.62 m aperture respectively, is provided with a mounting by Gautier. The objectives are supported side by side in a tube of rectangular section, divided longitudinally by a sheet metal screen. The focal length of the visual objective is 16.16 m, while that of the photographic objective The mounting is provided with the usual adjustments. The telescope stands on a massive pier at a great height above the ground in the reconstructed chateau of Meudon.

The equipment of the Observatory includes the elaborate apparatus employed by M. Janssen in his studies of the absorption spectra of gases at various pressures and temperatures; a reflecting telescope of Im aperture and 3 m focus; and the photoheliographs with which have been obtained the remarkable photographs of the Sun described in the remaining portion of the volume.

M. Janssen's memoir on solar photography opens with a brief historical sketch of the work of Draper, de la Rue, Rutherfurd and others. The first astronomical photograph, one of the Moon taken only a few months after the publication of Daguerre's discovery, was due to William Draper. The Sun was first photographed by Fizeau and Foucault in 1845. On the advice of Sir John Herschel solar photography was first taken up systematically at Kew in 1858, where Warren de la Rue succeeded in securing a daily series of photographs having a diameter of 0.10 m, which were well adapted for the statistical study of Sun-spots. On account of the small size of the solar image these plates did not bring out the granulation of the photosphere and the details of the spots. The photographs of Rutherfurd and Vogel were more servicable for this purpose, and showed many important details. But it was reserved for M. Janssen to perfect at Meudon a photographic process capable of revealing the structure of the photosphere as it is rarely seen with powerful telescopes, employed under the best atmospheric conditions. This work was undertaken in 1874, in connection with the experiments on the use of photography made in preparation for the transit of Venus of that year.

The Meudon photoheliograph consists of an objective of 0.135 m aperture, by Prazmowski, mounted at the end of an adjustable wooden tube supported by a carriage on rollers. In the focal plane of the objective is a rapid exposing shutter, consisting of an adjustable slit in a metal plate which can be shot by springs across the axis of collimation. An amplifying lens mounted just beyond the exposing shutter gives an enlarged image of the Sun on a wet collodion plate at the lower end of the camera tube.

With this apparatus photographs of the Sun on a scale of 0.20 m, 0.30 m, 0.50 m and in a few cases 0.70 m to the Sun's diameter have been obtained. The diameter ordinarily employed is 0.30 m, as images of this size have been found to be most generally useful. M. Janssen believes the excellence of the photographs may be ascribed to:

1. The use of objectives made of selected glass, and achromatized