450 Scientific Proceedings, Royal Dublin Society. 
The lunar photographs taken with the 13-inch reflector at 
the University Observatory, Oxford, have been utilized for find- 
ing a new value for the semi-diameter of the Moon.* The 
process adopted was very nearly the same as that employed 
by Wichman, and the resulting mean semi-diameter is 
15’ 34”:175 +0069. Mr. Neison has, from a careful discussion 
of nearly 1100 observations, made at Greenwich, Oxford 
(Radcliffe), and Washington deduced the value— 
1b’ 33°37 + 4”10+(1 + 070 x aperture in inches). 
This empirical formula, he adds, agrees closely with the 
theoretical formula, founded on the assumption that the differ- 
ences between the semi-diameters obtained with instruments of 
different aperture vary as the diffraction discs and the amount 
of light. Applying this formula to a 13-inch reflector, and 
adding the photographic irradiation, which Mr. Neison states his 
experiments indicate to be about +08, the photographic semi- 
diameter should be 15’ 34°-08.t 
6. The Intra-Mercurial Planet Question. 
A new attempt to find an orbit in accordance with the more 
or less doubtful observations of dark round spots passing across 
the Sun’s disc, was published in the A. N., by Herr v. Oppolzer. 
He found a system of elements which was in remarkably good 
accordance with the eight observations on which the calculations 
were founded. Watson’s observation, during the eclipse in 1878, 
was not among these, and the resulting orbit made “ Vulkan ” at 
that moment be 7° preceding the Sun. A nearly central transit 
ought to have taken place on March 18, 1879, but nothing was 
seen, though many telescopes were directed to the Sun that 
day. 
In No. 2258-54 of the A. N. Dr. C. H. F. Peters has published 
a long article entitled, “ Some critical Remarks on so-called intra- 
Mercurial Planet Observations.” In the first part of this article 
the writer considers at length the observations made during 
the eclipse of July 29, 1878, by Watson and Swift, of two 
unknown objects south-west of the Sun. The fact that the line 
between the two stars, called a and b by Professor Watson, is 
* M, N. xxxix., p. 447, + M.N xl, p. 6. 
