ASTRONOMY. 'Ml 



tiou, each having about the same focal length (3-GO meters), and aper- 

 tures of 0-24 meter and 0-34 meter respectively. The latter telescope, 

 which serves for the photography, is rendered achromatic for chemical 

 rays ; the other telescope is used as a pointer in keeping the image of 

 the star at exactly the same point of the photographic plate for the 

 requisite length of time; the field is about 3°. The Messrs. Henry 

 have recently succeeded in obtaining, with an exposure of one hour, a 

 very beautiful proof, which has been presented to the Academy by M. 

 Mouchez. It represents an expanse of about 5° and shows 2,790 stars 

 between the 5th and 14th magnitudes. Such a chart as this one, ob- 

 tained in one hour, would have certainly required a number of months 

 to ])repare by ordinary methods." (Bull. Astron., June, 1885.) 



Fhotographic charts of regions of the Mill'y Way, by Paul and Prosper 

 Henry, of the Paris Observatory. — "About 5,000 stars between the 

 6th and 15th magnitudes can be counted on the plate presented to the 

 Academy by M. Mouchez. The plate shows an area of 2° 15' of right 

 ascension by 3° of declination, and was obtained by the new photo- 

 graphic apparatus described in vol. ii, p. 289, of the Bulletin xbjtro- 

 nomique. In order to avoid confounding flaws in the plate with stars 

 three successive exposures of an hour each were made, the telescope 

 being moved 5" each time, so that each star is represented by three 

 points forming an equilateral triangle, the sides being 5" in length. 



"M. Mouchez estimates that 0,000 similar plates would be required to 

 represent the whole sky, and that these would form 1,500 charts of the 

 size of the ecliptic charts of the Paris Observatory." {Bull. Astron.j 

 August, 1885.) 



Photograph of the star cluster of Perseus. — " During September, 1884, 

 Dr. Lohse took advantage of exceptionally clear nights to attempt some 

 star photographs, using the 11-inch refractor belonging to the Potsdam 

 Observatory. An exposure of 45 minutes generally sufficed for obtain- 

 ing photographs of stars as faint as 10th to 11th magnitude. The finder 

 of the telescope was employed as a pointer in correcting the errors due 

 to the driving clock; but it would have been better to point the telescope 

 directly by means of an eyepiece inserted in the photographic cham- 

 ber. Such an arrangement has been invented by Mr. Common. Two 

 successive positions of the most brilliant star of the group have always 

 been reproduced for the purpose of fixing upon the plate the position of 

 the apparent parallel. The sensitive plates were furnished by Wratten 

 and Wainwright of London. The images were developed by a concen- 

 trated solution of oxalate of iron, to which was added potassium bro- 

 mide. The develoi)ment can be hastened b^^ plunging the plates for 

 about two minutes into a very weak solution of nitrate of chrysaniline. 



Dr. Lohse has succeeded in this way in photographing a certain num- 

 ber of star clusters, and in particular the cluster;^ Persei, whicli Dr. H. 

 C. Vogel studied in 1878. A negative obtained Sei)tember 24 served 

 to identify the stars by micrometer measures, and one obtained Sep- 



