ASTRONOMICAL PROGRESS AND PHENOMENA. 



Prof. Pritchard lays stress upon the fact that 

 each plate must be considered as carrying with 

 it its own scale ; and, due regard being paid 

 to the unavoidable though slight variations of 

 scale in the different plates, he is of opinion 

 that in this delicate class of work photography 

 will give as accurate results as any other known 

 method. The nebula No. 1180 of Herschel's 

 " General Catalogue " has been photographed 

 by Mr. Roberts in England, and by the Henry 

 Brothers in Paris. The latter made their neg- 

 ative on Jan. 27, 1887, giving it an exposure 

 of two hours, and obtaining an image extend- 

 ing 25' from east to west, and 15' from north 

 to south, with a nebulous star, or condensa- 

 tion, a little detached toward the south. Dur- 

 ing such long exposures as this it is necessary 

 to guard against displacements of the images 

 upon the sensitive plate, arising from changes 

 in the refraction due to the earth's diurnal mo- 

 tion. Dr. Dryer has investigated the magni- 

 tude of these displacements, and finds that in 

 latitude 50 north, with a perfect clock, and 

 an accurately adjusted instrument, they will 

 not exceed 0'5" (and may therefore be neg- 

 lected) in the case of an equatorial star moving 

 from 27 m east to 27 m west of the meridian, or 

 in the case of a star of +25 declination mov- 

 ing from 39 m east to 39 m west of the meridian. 



Prof. E. C. Pickering has recently published 

 his first annual report on the photographic study 

 of stellar spectra at the Harvard College Ob- 

 servatory, made with funds provided by Mrs. 

 Draper as a memorial to her husband, the late 

 Dr. Henry Draper. The results already obtained 

 are so encouraging that Mrs. Draper has de- 

 cided to extend the original plan of the work 

 sufficiently to embrace a complete discussion of 

 the constitution and conditions of the stars as 

 revealed by their spectra, so far as present sci- 

 entific methods permit. In order to reach all 

 parts of the sky, it is expected that a station 

 will be established in the southern hemisphere. 

 The investigations already undertaken extend 

 only to declination 24, and include a cata- 

 logue of the spectra of all stars of the sixth 

 magnitude and brighter, a more extensive cata- 

 logue of spectra of stars brighter than the eighth 

 magnitude, and a detailed study of the spectra 

 of the bright stars. Three telescopes have been 

 used in the work : an 8-inch Voigtlander pho- 

 tographic lens refigured by Alvan Clark & 

 Sons, Dr. Draper's 11-inch photographic lens, 

 and the 15-inch refractor of the Harvard Ob- 

 servatory. The spectra have been produced 

 by returning to Fraunhofer's method of placing 

 a large prism in front of the object-glass. The 

 spectrum of a star formed in that way is ex- 

 tremely narrow when the telescope is driven 

 by clock-work in the usual manner, but it may 

 be broadened aa much as is desired by giving 

 the telescope a speed slightly different from 

 that of the earth. The negatives were sub- 

 sequently enlarged ; and a specimen print is 

 given of part of the spectrum of Pollux on a 

 scale that would separate II and K by nine- 



tenths of an inch. In a comparatively short 

 space it shows nearly 150 lines with remark- 

 able clearness. Several points of interest have 

 been already brought out by Prof. Pickering's 

 work. A photograph of a Cygni, taken Nov. 

 26, 1886, shows the #line double, its two com- 

 ponents having a difference in wave-length of 

 about one ten-millionth of a millimetre. A 

 photograph of o Ceti shows the lines G and h 

 bright, as are also four of the ultra-violet lines 

 characteristic of spectra of the first type. The 

 If and Klines in this spectrum are dark, show- 

 ing that they probably do not belong to that 

 series of lines. The star near % l Orionis, dis- 

 covered by Gore in December, 1885, gives a 

 similar spectrum, which affords additional evi- 

 dence that it is a variable of the same class as 

 o Ceti. Spectra of Sirius show a large number 

 of faint lines, besides the well-known broad 

 lines. Progress is reported in the various in- 

 vestigations that are contemplated, namely : 

 1. Catalogue of spectra of bright stars. 2. Cata- 

 logue of spectra of faint stars. 3. Detailed study 

 of the spectra of the brighter stars. 4. Faint 

 stellar spectra. 5. Absorption spectra. 6. Wave- 

 lengths. Mrs. Draper has decided to send to 

 Cambridge a 28-inch reflector and its mount- 

 ing, and the remarkably perfect 15-inch mirror 

 constructed by Dr. Draper and used by him in 

 making his photograph of the moon. 



The literature respecting the use of photog- 

 raphy in astronomy has recently been enriched 

 by articles written by Mr. A. A. Common; Dr. 

 David Gill, Astronomer Royal at the Cape of 

 Good Hope; Prof. E. S. Holden, Director of 

 the Lick Observatory ; Admiral Mouchez, Di- 

 rector of the Paris Observatory; M. G. Rayet, 

 Director of the Bordeaux Observatory ; Otto 

 Struve, Director of the Pulkowa Observatory ; 

 and Prof. C. A. Young, of the Princeton Observ- 

 atory. Some of these articles are mainly histori- 

 cal, others are theoretical and practical, and 

 still others are of a more popular character. 

 Among the historical articles those by M. 

 Rayet, in the " Bulletin Astronomique," are 

 particularly noteworthy on account of their 

 completeness and accuracy. 



The Paris International Astropliotographlc Con- 

 gress. In compliance with the request of Ad- 

 miral Mouchez, an International Congress of 

 Astronomers was called in April by the French 

 Academy of Sciences, to take steps to obtain 

 a photographic chart of the heavens upon the 

 co-operative plan so successfully carried out 

 by the German Astronomische Gesellschaft in 

 their zone observations. The Congress was 

 opened at the Paris Observatory on April 16, 

 by M. Flourens, Minister of Foreign Affairs of 

 the French Government, and addresses were 

 made by M. Bertrand, the eminent mathema- 

 tician, by Admiral Mouchez, Director of the 

 Paris Observatory, and by Prof. Struve, Di- 

 rector of the Pulkowa Observatory. Admiral 

 Mouchez was chosen honorary president; 

 Struve, president; Auwers, Christie and Faye, 

 vice-presidents; Bakhuyzen and Tisserand, 



