her. Of the nineteen new stars, known to have appeared during the progress of this work, she discovered ten, and five more were found by other observers here. In this work also, the number of stars of the peculiar class known as fifth type, has been increased from seventeen to one hundred and eight.

For a more detailed study of the bright stars, prisms have been attached to the 11-inch Draper Telescope at Cambridge, and to the 13-inch Boyden Telescope at Arequipa. Spectra of the brightest stars have thus been obtained, six inches long, and half an inch wide, showing at least five hundred lines. Prisms twenty-four inches in diameter have been used with the Bruce Telescope in Arequipa, and sixteen inches in diameter with the Metcalf Telescope in Cambridge. The latest and largest investigation undertaken here, as part of the Henry Draper Memorial, is a catalogue giving the class of spectrum of a hundred thousand stars of the eighth magnitude and brighter, shown on the photograph taken with the 8-inch doublets. The classification of spectra used in the Draper Memorial, has been accepted by the superintendents of the principal nautical almanacs in their standard catalogue of three thousand stars, and also at the leading observatories. The preparation of the catalogue mentioned above has been undertaken by Mrs. Fleming's successor, Miss Annie J. Cannon, who has devoted a large part of her time during the last fifteen years to the detailed study of stellar spectra. Her classification of one thousand stellar spectra published in Volume 28 of the Harvard Observatory Annals, occupied her for three years. To complete, in a reasonable time, a catalogue of one hundred thousand spectra evidently required the most careful study of the methods of "scientific management." As a first step, her contribution to the work, which required the greatest skill, was reduced from one hundred to six per cent., the remainder being performed as a great piece of routine work, by less experienced assistants. The utmost care has been taken to maintain the highest degree of accuracy, the probable error of the result for each star being about a tenth of one interval, corresponding to four one-hundredths of a stellar magnitude. Miss Cannon is now classifying five thousand spectra a month, and has

already classified twenty-seven thousand spectra. The completed catalogue will fill four of the quarto volumes of the Annals of the observatory, of about two hundred and fifty pages each, and will give the class of spectrum of nearly all of the stars of the eighth magnitude and brighter, besides many others. Some of these are so faint that they are not contained in the Cape Photographic Durchmusterung.

Dr. Draper thus placed in our hands a wonderful tool for analyzing the stellar universe. His memorial furnishes not only a permanent record in print of great extent, but, through the collection of photographs, will permit in the future a vastly greater number of facts to be derived from them.

PROC. AMER. PHIL. SOC., LI. 207 J, PRINTED JAN. 17, 1913.

NOTES ON PLATES OF NOVA GEMINORUM OF 1912 TAKEN WITH THE BRUCE SPECTROGRAPH

OF THE YERKES OBSERVATORY.

BY STORRS B. BARRETT.

(Read April 20, 1912.)

1912, March 15. Displacement of sharp H and K lines indicates a velocity of 17 km. recession. This velocity does not differ greatly in all the photographs. Many titanium lines are represented by absorption lines displaced about 7 A. toward the violet. The relative brightness of the emission components of the hydrogen lines ß, 7, 8, as compared with the continuous spectrum is less than in all subsequent plates.

March 21. The centers of the bright bands are now 2 Å. toward the red, as compared with 10 Å. on March 15. Note the sharp bright line at a 4,526. A second absorption band now accompanies each bright band.

March 24. Shows marked increase in intensity of the second absorption band for each hydrogen line.

March 29. Two bright maxima may be seen near the red edge of the bright HS band. They are also present in He. A similar line near the beginning of the bands is more difficult to see on the print.

March 30. There are two conspicuous bright maxima near the beginning of the bright Hy band and one or two near the red edge. On a short-exposure plate of April 1 three conspicuous maxima are seen near the red edge of ẞ, y and 8.

April 1 and 2. The hydrogen bands are concentrating their intensity toward the red edge. Note that there are two bright superposed bands for each hydrogen line, one much longer and fainter than the other. This is first indicated plainly on March 29. The broad bright band at λ 4,640 has been gradually gathering intensity. YERKES OBSERVATORY,

WILLIAMS BAY, WIS.,

April 18th, 1912.

RELATIONS BETWEEN THE SPECTRA AND OTHER CHARACTERISTICS OF THE STARS.

BY HENRY NORRIS RUSSELL.

(Read April 20, 1912.)

To the student of the stars, who attempts to arrange our existing knowledge in such a manner that some light may be thrown upon the problems connected with stellar evolution, the spectral classification developed at Harvard is of vital importance.

In such investigations, we must deal, if possible, not with single instances, but with representative averages for groups of stars. But really representative averages are often much harder to obtain. than might be supposed. Consider, for example, the actual brightness of the stars. We can find this only when we know the distance of the star-and out of the hundreds of thousands of stars which have been catalogued, we know the distance of barely five hundred. But even if we knew the exact distances of the 6,000 or more stars which are visible to the naked eye, we would not have a fair sample of the general run of stars. To explain how this may happen, let us suppose that there were only two kinds of stars, one equal to the sun in brightness, and the other 100 times as bright as the sun, and that these were distributed uniformly through space, in the proportion of 100 stars of the fainter kind for every one of the brighter. To be visible to the naked eye, a star of the fainter sort must lie within about 55 light-years from the sun; but all the stars of the brighter kind which lay within 550 light-years would be visible. We would therefore be searching for these stars throughout a region of space whose volume was 1,000 times greater than that to which our method of selection limited us in picking out the fainter ones, and our list of naked-eye stars would consequently contain ten stars of the brighter kind to every one of the fainter-though if we could select instead the stars.

contained in a given region of space, we would find the disparity to be 100 to I the other way.

It is therefore a fortunate circumstance that the stars whose distances have been measured have for the most part been chosen, not on account of apparent brightness, but because of relatively rapid proper-motion-which is found by experience to be a fairly good indication of actual nearness to our system. These stars, therefore, represent mainly the sun's nearer neighbors, without such an egregious discrimination in favor of stars of great actual brightness as we have seen must occur if we choose our stars by apparent brightness alone. Some traces of this discrimination will still be unavoidable, for our knowledge of the proper-motions of the fainter stars is still imperfect, and stops short at a little below the ninth magnitude.

In addition to the stars whose parallax has been directly observed, we have data for many more, which belong to clusters whose distances have been found by combining data regarding their proper-motions and radial velocities. In this case too the absence of proper-motion data (which decide whether or not a star really belongs to the cluster) prevents us from obtaining information about stars fainter than a certain limit; but otherwise our knowledge is probably fairly complete.

In the present discussion of the relation between the spectral type and the real brightness of the stars, those directly measured parallaxes have been employed which are confirmed by the work of two or more observers, and also a few results obtained by single observers whose work is known to be of high accuracy, and free from sensible systematic errors. To these have been added the members of the Hyades, the Ursa Major group, the "61 Cygni group" and the moving cluster in Scorpius discovered independently by Kapteyn, Eddington, and Benjamin Boss. The spectra of a very large number of these stars have been determined at Harvard especially for this investigation, and the writer takes pleasure in expressing his most hearty thanks to Professor Pickering and Miss Cannon for this generous and invaluable aid.

The actual brightness of the stars may best be expressed by