ASTRONOMICAL PROGRESS AND DISCOVERY. 



49 



of the same aperture, but of different focal 

 lengths. He found that mirrors, particularly 

 those of short focal length, are comparatively 

 unsuitable for the photographic work of chart- 

 in;.' the heavens. 



At the Paris Observatory M. Loewy's new 

 method for determining aberration and refrac- 

 tion is being used. The brothers Henry have 

 continued their magnificent work in celestial 

 photography, having taken seventy-four plates 

 of different parts of the sky in 1887. The re- 

 port of the director, Admiral Mouchez, con- 

 tains an engraving of the Pleiades made up 

 from three of the Henry protographs. 



Astronomical Photography. Prof. Pritchard. of 

 Oxford Observatory, was encouraged by his 

 success in determining from photographic plate? 

 the parallaxes of the components of 61 Cygni, 

 to discuss the parallaxes of p. Cassiopeia and 

 the pole-star. His equatorial he improved, 

 and on each of fifty-three nights four plates 

 were taken of p. Cassiopeia?. The exposures 

 varied from five to ten minutes. About three 

 per cent, of the plates were injured or unsuit- 

 able for measurement. He took two impres- 

 sions on the same plate, slightly moved in 

 position. Two comparison-stars were used. 

 The resulting parallaxes were : 



From star (A) ir - O'OSOl" O'i>27". 

 " (B) IT = 0-0:211 0-02:35. 



An investigation of the results obtained by 

 using only a few selected plates of 61 Cygni 

 and p. Cassiopeia? has led Prof. Pritchard to 

 give up the laborious method used in the case 

 of 61 Cygni, and hereafter to limit the observa- 

 tions to five niglits in each of the four periods 



of the *ear indicated by the position of the 

 parallalfcc ellipse. He hopes in this way to 

 determine in one year the parallaxes of fifteen 

 stars. He plans to apply this method - 

 matically to all stars between the magnitudes 

 1-5 to 2-5 which are well visible at Oxford. 

 From a discussion of the approximate paral- 

 laxes that he expects to obtain, Prof. Pritch- 

 ard hopes to infer some important cosmical 

 relations. The result of his approximate de- 

 termination of the parallax of Polaris is TT = 

 0-052". A mean of all the determinations of 

 preceding observers is, according to Maxwell 

 Hall, TT = 0-043". From six months' observa- 

 tions, Prof. Pritchard has obtained the follow- 

 ing provisional parallaxes : 



o Cassiopeia?. 0'072" 0-42". 

 /3 Cassiopeia?, 0-157 " 

 t Cassiopeia, O'OoO 



Isaac Roberts has taken photographs of 

 the ring nebula? in the Lyre (57 M. Lyra?), the 

 dumb-bell nebula* (27 M. Vulpeculae)," and the 

 fine, globular star-cluster (13 M. Herculis). In 

 the first the ring was well shown, also the 

 central star and nebulous matter in the in- 

 terior, but there was no evidence of resolva- 

 bility. The photographs seem to confirm the 

 suspicion that the central star is variable. 

 Photographs of the star-cluster showed promi- 

 nent features not noticed by Sir J. Herschel and 

 VOL. xxvin. 4 A 



the Earl of Ross. All these photographs were 

 enlarged from three to twenty-five times. Mr. 

 Eoberts calls attention to the important fact 

 that, owing to different causes, which are not 

 easily discernible, but may be atmospheric, 

 chemical, and mechanical, the same area in the 

 heavens will show, on the same exposure with 

 similar plates, with apparently the same clear- 

 ness of sky, surprising differences in the num- 

 ber of stars. He finds, on comparing MM. Hen- 

 ry's plate of the stars in Cygmis taken in 1885, 

 with those taken by himself in 1886 and in 

 1887, that the number of stars in the Henry 

 plate is 3,124-: in his plate of 1886, 5,023; and 

 in his plate of 1887, 16,206 ; the exposure in 

 each case was sixty minutes. The brothers 

 Henry have succeeded in taking a photograph 

 of the Pleiades after an exposure of four hours, 

 which shows very much more nebulous mat- 

 ter than their well-known photograph taken 

 last year. The negative shows stars down to 

 the seventeenth magnitude. 



Photographic (hart of the Heavens. Dr. Gill, at 

 the Cape of Good Hope, is pushing this work 

 in its preliminary stages with great energy. 

 The photographic instrument is kept at work 

 by two observers from evening twilight until 

 dawn. The reduction of the plates from south 

 polar distance to 12'5 has been com- 

 pleted, and measurements are proceeding to 

 south polar distance 30. Derby dry plates 

 were used with half-hour exposure, instead of 

 an hour as previously. Dr. Gill, in a paper 

 published by the International Committee for 

 the Photographic Charting of the Heavens, 

 proposes the establishment of a central bureau 

 consisting of chief, assistants, secretaries, and 

 a staff of measurers and computers to take the 

 photographs and measure them and make a 

 catalogue, the work to go on for twenty-five 

 years, at a cost of $50,000 per annum. This 

 would require the cataloguing of 2,000,000 

 stars. Some astronomers object to this work 

 as being unnecessary. It is expected that a 

 considerable number of observatories in Europe 

 and America will begin work on the photo- 

 graphic chart in 1889. 



Solar Parallax. Prof. "William Harkness, in 

 No. 182 of the "Astronomical Journal," gives 

 an abstract of his paper, read before the A. A. 

 A. S.. " On the Value of the Solar Parallax 

 deducible from American Photographs of the 

 Last Transit of Venus." In this paper an ac- 

 count was given of the instruments and pro- 

 i.-mployed by the United States Transit 

 of Venus Commission in determining the solar 

 parallax from photographs of the transit of 

 Venus which occurred in December, 18S2. 

 Let TT be the solar parallax, and 8A and 5D, 

 respectively, the corrections to the right as- 

 censions and declinations of Venus given by 

 Hill's tables of that planet. Then, on the 

 assumption that Hansen's tables of the sun are 

 correct, there resulted from measurements of 

 the distances between the centers of the Sun 

 and Venus, made upon 1,475 photographs, 



