382 
Vienna telescope, the pointing in right ascen- 
sion could be changed by a number of seconds, 
so as to throw an object entirely away from 
the wire. The main question is, however, the 
steadiness of motion when no pressure what- 
ever is applied by the observer or the wind; 
and, so far, I have found no large telescope 
which is entirely satisfactory. The Vienna 
telescope was not supplied with a micrometer 
at the time of my examination, so that I could 
not test its motion as thoroughly as I wished to ; 
but, by bringing the planet Uranus in the edge 
of the field, I found that there was constantly 
an irregular movement in right ascension, the 
amount of which I estimated as between one 
and two seconds of are. This movement had 
no regular period, and therefore did not seem 
to be connected with any defect in the figure 
or motion of the screw. Its irregular period, 
if I may use the term, varied from the small- 
est appreciable amount to two or three seconds 
of time. Its most probable cause seemed to 
be the irregular friction of the motion in right 
ascension, and especially of the friction-rollers, 
by which the polar axis is supported at its 
lower end. A similar irregularity is noticed 
in the Washington telescope; but I think it 
is decidedly less than in the Vienna one, 
provided that no strong wind is blowing on 
the instrument. 
Arrangement of sector.—In Mr. Grubb’s 
large telescope an attempt is made to give 
greater stability to the screw by having its 
axis immovably fixed to supports in the massive 
base of the telescope, which renders it inca- 
pable of any motion except that of turning. 
The screw cannot, therefore, be unlocked from 
the sector, as in the instruments by other 
makers. When the sector reaches the end of 
its motion, it is to be turned back by giving a 
rapid backward motion to the screw itself, for 
which special apparatus is provided. From 
what I have already said, I am of opinion that 
this arrangement offers no advantage to com- 
pensate for the trouble which it causes the 
observer. 
Slow motion. —The slow motion in right 
ascension in the Vienna telescope is endless, 
Instead of being confined between narrow limits, 
as in that at Washington. 
improvement, saving the observer much loss 
of time from the motion running out. 
Illumination. — The apparatus for illumi- 
nating the field of the micrometer was not in 
perfect order at the time of my visit, so that 
I need not report upon it in this connection. 
It is in its general character similar to the 
system adopted by the Messrs. Repsold, of 
SCIENCE. 
This is a decided 
which I shall speak hereafter. The illumina- 
tion of the divisions of both circles leaves 
nothing to be desired. 
Minor points. —In the preceding I have 
indicated what may be considered fundamental 
points affecting the use of the instrument. 
There are, however, several minor points 
which are of almost equal importance, so far 
as the practical use of the instrument is con- 
cerned. As the instrument now stands, the 
drawback which strikes me most was the 
absence of any rough setting, either in right 
ascension or declination, and the impossibility 
of seeing, even approximately, the pointing 
in declination, except when the observer is at 
the eye-piece. This, when combined with the 
great force necessary to move the telescope in 
declination, makes its pointing a difficult and 
troublesome operation. The observer must 
first set the telescope by pure guess-work. He 
has then to mount to the eye-piece, wherever. 
it may be, look into the microscope, and note 
the reading of the circle. He has then to with- 
draw his eye, and, by considerable muscular 
exertion, to make another guess, which he can 
test by again reading the circle. ‘Thus: the 
pointing is to be made by a series of trials, 
which are so troublesome that I found the 
observers were in the habit of mounting to the 
top of the clyinder in the dome, and finding 
the pointing in declination by moving the 
telescope around the horizon. 
I remark, in this connection, that the Wash- 
ington telescope has a coarse setting, which 
the observer can read from any point below the 
telescope with the aid of an opera-glass. 
Objective. — The proper figuring of a great 
objective so as to give the best possible image 
of a celestial object is justly considered the 
most difficult task in the construction of a large 
telescope : especial interest, therefore, attaches 
to Mr. Grubb’s success with the objective. The 
atmospheric conditions were very unfayorable 
to the finest. tests, but I succeeded in making 
such examination as the circumstances admit- 
ted on three evenings. On the first trial the 
image was found to be distorted, owing to want 
of adjustment of the glass itself. This was 
soon corrected by Director Weiss. On the 
second trial I found a well-marked spherical 
aberration, which seemed, however, to be very 
regular from centre to circumference. But 
there had been a fall of temperature, and the 
dome had been opened but a short time, — 
circumstances under which the Washington 
telescope always exhibited the same phenom- 
enon. On the third evening the dome had 
been opened long enough to nearly equalize 
[Vou. ILL., No, 60. 
; 
f 
