500 
EARL ROSSE’S OBSERVATIONS ON THE NEBULA. 
axis of motion parallel to the axis of the earth, the movement of the telescope can be 
rendered almost exactly equatorial : there was some mechanical advantage in placing 
the pulley a little out of that line ; and for such measurements as we have required, 
we have found the movement of the telescope sufficiently equatorial without the sub- 
sidiary apparatus, and therefore have not up to the present time made use of it. 
When the telescope is in the meridian, as it moves in polar distance it is guided 
by a cast-iron arc of a circle about 85 feet diameter nicely planed. The arc is 
composed of pieces 5 feet long, each adjusted independently in the meridian by the 
transit instrument, and secured to massive stonework. The horizontal axis of the 
great universal joint gives motion to an index which points to polar distances on an 
arc of 6 feet radius, by which the telescope is very quickly set in polar distance. A 
20-inch circle with a very delicate level, attached to the telescope, performs the same 
office, more slowly but with greater accuracy ; and also gives polar distances with 
considerable precision when duly corrected. The whole mounting was planned espe- 
cially with a view of carrying on a regular system of sweeping, for which it is pecu- 
liarly adapted; but the known objects which require examination are so numerous 
that hitherto we have been fully occupied with them ; and the discovery of new 
nebulae has as yet formed no part of the systematic work of the observatory. 
As yet the telescope is not provided with a clock movement. A clock movement 
was part of the original design, and there would have been no serious difficulty in 
carrying it out ; but the want of it has not been very much felt, and there were other 
matters requiring more immediate attention. 
Various micrometers have been tried, but upon the whole the common wire micro- 
meter with thick lines succeeds the best. The thick lines are formed by coiling very 
fine silver wire four times round the forks, soldering it there, and then removing the 
lower half of the coil. A little spirit varnish unites the fine wires into a thin ribbon 
with a straight edge, perhaps as perfect as can be made. The micrometer is used 
without illumination; and I have never failed to see the lines in the darkest night; 
but of course measurements with thick lines are inferior in point of accuracy to mea- 
surements with thin lines in an illuminated field. Unfortunately any micrometrical 
contrivance which either diminishes the light of the telescope, or renders the field less 
dark, extinguishes the faint details of the nebulae, which even with an aperture of 
6 feet are often barely perceptible. There have been many ingenious attempts to 
make fine lines visible in a perfectly dark field, but they have not, at least as far as 
my experience goes, been entirely successful. 
The telescope has two specula, one about three and a half, and the other a little 
more than four tons weight. Each speculum was originally provided with a system 
of levers to afford it an equable support : it was placed upon this system before it 
was ground, and it has rested upon it ever since. The system of levers is a combina- 
tion of three systems in every respect similar, resting on three points under the centres 
of gravity of the three equal sectors into which the speculum may be supposed to be 
