204 



ASTRONOMY. 



stated this. Mr. Swift has also replied in the A. N. 2277. He had, 

 immediately after the eclipse, informed the other observers near him 

 that he had seen two objects with sensible (though, he acknowledged, 

 £>purious) disks about 3° S. W. of the Sun, 12' apart. Later on he 

 changed this estimate to 1' or 8' in a letter to Nature (xviii, p. 539), in 

 which he made a curious mistake, putting 8' = 2™. Prof. Peters can 

 hardly be blamed for haviug felt suspicious at all this confusion, or for 

 having made little of Mr. Swift's comparison of the distance between 

 Mizar and Alcor, with the distance between the two unknown objects. 



Venus and Mars. — Dr. Hartwig has published an extremely import- 

 ant investigation of the diameters of Venus and Mars from heliometer 

 measures by himself. He has also thoroughly discussed previous ob- 

 servations, and the following instructive table gives the main results: 



A. — Heliometer. 



i. Prob. Error. Aperture. 



" " luin. 



1820 Brandes -1-093 J-0. 134 7(3 



1847-49 Wichmann -0-436 ' — 158 



1857 Wiunecke —1-088 — IG'i 



18r)l-(i3 Main -1-182 -091 190 



1876-77 Hartwig - —1-233 -057 76 



B. — Double Image Micrometer (Airy). 



i. Prob. Error. Aperture. 



" " mm. 



186-2-C5 Kaiser —0-772 -^-0 -042 189 



1840-52 Main -0-50? — 170 



1873 J. Plummer _ 0-546? — 152 



C. — Rochon's Micrometer. 



i. Prob. Error. Aperture. 



" " mm. 



1810-15 Arago -1-519 -^0 -105 162 



D. — Wire Micrometer. 



i. Prob. Error. Aperture. 



" " mm. 



1833-36 Miicller +0 -651 ^0 -073 97 ' 



1854-57 J. Schmidt j +2 '^]^ ^ "ij[l \ 108* 



1860-64 Miidlor -0 -254 — 244 



1838-39 Galle -0-86 — 244 



1871 Vogel - -2-56 — 293-5 



The negative sign of the constant error for all double image observa- 

 tions is very remarkable, and it appears that telescopes of all apertures 

 make the diameter as measured in daylight too small, while there does 

 not seem to be any law depending on the aperture. 



The adopted value is a mean of the Oxford, Leiden, and Strassburg 

 values, 17".552, which is 0".60 and 0".65 greater than the results of ob- 

 servations of Venus in transit, by Auwers and Tennant, in 1874. The 

 obscured hemisi3here of Venus was repeatedly seen during the obser- 

 vations. 



♦Powers, 120 and 90. 



