454 Scientific Proceedings, Royal Dublin Society. 
the darker background explains probably this phenomenon. In 
the case of the wire micrometer, the difficulty in making the 
contact of the sharply seen spider-line and the far less distinctly 
seen edge of the planet, must necessarily allow a far greater 
range for individual errors, and in the opinion of the author this 
micrometer is not to be recommended for diameter determinations. 
Jn conclusion, the mean of the results found at Oxford, Leiden, 
and Strasburg, 17552 is adopted as the most probable value. 
This result is 0°60 and 0”°65 greater than those found by Auwers 
and Tennant during the transit on December 8, 1874. 
In an appendix are given extracts from the observer's diary 
concerning the visibility of the un-illuminated part of Venus. 
The dark part was repeatedly seen in geocentric distances 
(Venus-Sun) ranging from 5° to 22°. 
Mars.—The second part of Herr Hartwig’s work contains his 
researches on the diameter of Mars. His and his predecessors’ 
measures were reduced in the same manner as those of Venus. 
The constant error is generally very small, and does not show any 
regularity. From Arago’s, Bessel’s, Kaiser's, Main’s, and Hartwig’s 
measures, the following result for distance unity is derived: 
Polar Diam. = 9”°349 + 0-010, or by simply taking the mean 
of the four determinations, 9’°352. 
The ellipticity of Mars has not yet been found in a satisfactory 
manner. The Strasburg observations gave the two diameters 
9’°311 and 9-519. Bessel found no difference whatever, but the 
other observers have ; js is the mean of Encke’s, Galle’s, and the 
above-named observers’ results. The opposition in 1879 was 
very favourable for the determination of the ellipticity, as well 
as of the position angle of the axis of rotation, as shortly after 
the culmination the polar and equatorial diameters made equal 
angles with the vertical, whereby personal errors depending on 
this angle would be eliminated. 
The very favourable opposition of Mars in 1877 was taken 
advantage of by many observers to make drawings of the surface 
markings. Above all, Mr. N. E. Green’s “Observations of Mars 
at Madeira” (Memoirs R.A.S., XLIV.), deserve to be mentioned. 
With a 13-inch Newtonian Mr. Green obtained at Madeira 
forty-one drawings during August and September (the opposi- 
tion took place on September 5), and twelve of these, giving 
