704 EAEL OF EOSSE ON THE CONSTEUCTION OE SPECULA OE 6-EEET APEETUEE, 
The little rough sketches in the margin are exact copies from the sketches made at 
the moment in the observing-book. There are usually several sketches of the same 
object made at different times ; we have endeavoured to select the best. 
As to the drawings, they usually represent the objects a little stronger than they 
appear on an ordinary night, but not stronger than on a fine night, when the ah’ is clear 
and the sky black. Most of them have been repeatedly compared with the objects by 
difierent persons, and some have been several times sketched independently ; so that I 
trust they are upon the whole accurate. The central portion of the nebula of Orion has 
been drawn with great care by Mr. Bindon Stonet, and Mr. Huntee has been engaged 
this season in finishing the remainder ; but another season will be required to complete 
the work. 
Although there is probably no remarkable object in the list which I have not several 
times examined myself, for the great bulk of the observations I am indebted to the 
gentlemen at the time in charge of the Observatory. Mr. Johxstoxe Stoney’s observa- 
tions commenced in July 1848 ; and in June 1850 he was succeeded by his brother, 
Mr. Bixdox Stonet. He continued in charge of the Observatory till May 1852; after 
that, Mr. Mitchell observed for about two years. 
Mr. Johnstone Stone y occasionally also worked with his brother, and sometimes with 
Mr. Mitchell. 
Though so many of the observations were made in my absence, they are not the less 
to be relied on : nothing was done by an unpractised hand, and no pains were spared 
to ensure accuracy. I refer with as much confidence to the observations of the two 
Mr. Stoneys and Mr. Mitchell as if I had on every occasion been present myself, be- 
cause I know that they had thoroughly mastered the instrument and the methods of 
observing before they recorded a single independent observation ; they were, besides, 
eminently cautious and painstaking. 
There are no micrometer observations by Mr. Mitchell : I now rather regret it, as 
several cases of suspected change have recently been brought to light in arranging the 
materials of this paper. The fault, however, was mine. It appeared to me so highly 
improbable that any change would be detected, that I requested Mr. Mitchell to press 
on and not spend time on the micrometer. The most remarkable case of suspected 
change is perhaps H. 1905. Heeschel gives a drawing of it, the axes of the two nebulse 
in a line. On April 11, 1850, Mr. Johnston Stonet remarks the two nebulae not in a 
line. April 17, 1855, Mr. Mitchell remarks the two nebulae are not in a line, but the 
axes are parallel, and gives a diagram. At the present time they are neither in a line nor 
parallel, but inclined at an angle of about sixteen degrees. The micrometer is employed 
without illumination ; various contrivances were tried for illuminating the lines in a dark 
field ; but the darkness was not absolute, and faint details were obliterated. We therefore 
substituted bars for lines. 
In the ‘ Transactions ’ for 1850 are given Mr. Johnston Stonet’s measures of H. 1622. 
M, Otto Steuve was good enough to send me measurements of the same spiral, and to 
