/ 
4 Observations of Nebule and Clusters of Stars. 
tube, and of the supports of the speculum under its great weight, exact absolute 
positions cannot be directly obtained, except where a known star is so near the 
object, that their position, angle, and distance can be measured. 
Into the text have been introduced diagrams, which are rough copies of those 
drawn at the telescope. They have been executed by the pantagraph process em- 
ployed by the Patent Type Founding Company for reproducing the weather charts 
for the daily newspapers. Some of the original diagrams have been re-made in more 
correct proportions with the aid of more recent measures (¢.g. 59-62, 202, &c., 276, 
655, and a few others.) Some more elaborate sketches have been reproduced by 
lithography, vide the Plates accompanying these observations. The observations 
include those made 
By Mr. W. H. Rambaut, Between Jan. 1848 and June 1848. 
,, Mr. G. Johnstone Stoney, 5 July 1848 ,, Aug. 1850. 
», Mr. Bindon Stoney, is Aug. 1850 ,, April 1852. 
, Mr. G. Johnstone Stoney, a Aug. 1852 ,,- Dec. 1852. 
» Mr. R. J. Mitchell, : Dec. 1853 ,, May 1858. 
» Mr. S. Hunter, 46 Feb. 1860 ,, May 1864. 
a IDYe, 18, (Si) IB 5 Feb. 1866 ,, Aug. 1867. 
» Mr. C. E: Burton, = Feb. 1868 ,, March 1869. 
» Dr. R. Copeland, i Jan, 1871 ,, May 1874. 
, Mr. J. L. E. Dreyer, Since Aug. 1874, by whom the greater part of the 
following paper has been prepared for the press. 
Though it forms no part of this paper to discuss the power and optical merits of 
the six foot reflector, as from time to time remarks have appeared, both from persons 
who had at some occasion or other had an opportunity of observing with that instru- 
ment, and also from others who have never been at Parsonstown, it may not, per- 
haps, be out of place just to draw attention to the circumstance, that, unlike the case 
of a refracting telescope, of the performance of which a competent Judge may be able 
to give an opinion of permanent value, the reflector of this year may be as to de- 
fining power practically a totally different instrument from what it may be in the 
next. The large speculum of the 6-foot has, with one exception, been repolished at 
least in every second year, generally every year, and at first, when great efforts were 
being made to push its power to the utmost, and to improve the process of polishing, 
much oftener. Every time the speculum is removed from the tube and repolished, 
the old figure, whether it be good or bad, is lost in the process, and a new one 
formed, whose merits in no way depend on those of the last, and the telescope, though 
in mechanism the same, is optically speaking a new one. It would be exceedingly 
difficult to give an estimate of its qualities in various seasons, and in the great 
majority of cases the value of an observation has been limited by bad atmospheric 
conditions rather than by imperfections of the instrument. 
Before polishing a six-foot speculum it was my late father’s practice to polish a 
three-foot one one or more times, until on testing it on a watch-dial attached to a 
pole fixed vertically over the machine, its performance was considered satisfactory. 
