406 EEPORT — 1873. 



ou May 23 and June 9, 1868, probably during twilight, (Astronomical Ee- 

 gister, No. 68, p. 181.) 



17. In the same year I was observing Venus attentively for some months, 

 chiefly in broad daylight, with a small but good achromatic. I saw spots on 

 different occasions ; and on July 4, 1868, at 1 p.m., I could see traces of the dark 

 disk, though unsteadiness of the air and insufficient optical power prevented 

 ine from becoming certain of what I saw. 



18. On February 5, 1870, the dark side of the planet, then near inferior 

 conjunction, was seen (in daylight, I suppose) by Mr. R. Langdon, of Sil- 

 verton, Devonshire. (Month. Not. vol. xxxii. p. 307; Astron. Reg. No. 115, 

 p. 163, where the year is erroneously stated to be 1872.) 



19. Captain W. Noble, of Leyton, Essex, saw the dark part of Venus very 

 distinctly on February 22, 1870, only twenty-four hours before conjunction, 

 in close proximity to the sun. In a later commitnication, Captain Noble adds 

 that he saw the dark side always darker than the surrounding sky, and that 

 he rarely failed to see it whenever Venus was in or near inferior conjunction. 

 (Month. Not. vol. xxx. p. 1-52 ; Astron. Reg. No. 88, p. 74, and No. 130, p. 2.58.) 



20. At the meeting of the Royal Astronomical Society, March 11, 1870, 

 Mr. Browning stated that, without any special contrivance, he could see all 

 the globe of the planet in his 12-inch speculum — perhaps on twenty different 

 evenings, as Mr. Browning told me orally, and always in bright twUight. 

 The unillumined side appeared darker than the sky around it. (Ast. Reg. 

 No. 88, p. 74, and No. 131, p. 281.) 



21. On August 9, 1870, I was regarding Venus in bright sunshine at 

 11 A.M., when a lady who was with mo at that time immediately perceived 

 the whole disk of the planet. I showed to her Schriitcr'a drawing, which 

 she declared to be in perfect accordance with what she saw in the telescope. 

 I fancied only at moments that I saw a faint lino of light all round tho 

 greyish disk. Illumination unusually large (0-35) ; air much disturbed at 

 the time. 



22. Dr. A. "Winnecke, of Karlsruhe, saw the phenomenon twice, on Sep- 

 tember 25, 1871, at noon, and November 6, 1871, at 5 a.m. (Astron. Nachr, 

 No. 1863, and No. 1866, vol. Ixxviii. pp. 236 & 287.) 



On the day subsequent to Dr. Winnecke's first observation, September 26, 

 Captain Noble could not make out the dark hemisphere so well seen by 

 him a year before that time, but he adds that the sky was not clear. (Month. 

 Not. vol. xxxii. p. 17.) 



From the above conspectus it appears that the unillumined side of Venus 

 has been seen by 22 different observers : — 



In twUight by 13 (once by 4, many times by 9), 

 In daylight by 11 (once by 6, many times by 5). 



4 observers saw a faint line of light encircling the dark disk, 19 of them saw 

 the disk itself. Of the 22 cases reported, 12 have been observed during the 

 last eleven years, say one per year ; and I am disposed to think that tho 

 phenomenon is a normal one, and that with sufficient optical power and 

 attention under a favourable sky it is to be seen at every inferior conjunc- 

 tion, though I would by no means advance that it is constantly visible, 

 which would be a statement directly opposed to facts. 



For the explanation of this remarkable j)henomenon the following causes 

 have been suggested : — 



1. Plwspliorescence. — This was the idea of Sir William Herschel, Harding, 

 and partly of Schroter. It docs not aj^pear clearly whether they under- 



