MARCH 28, 1884.] 
the temperatures. So far as I could judge, 
the character of the image was perfect, there 
being no appearance of those rings of differ- 
ent focal lengths which are commonly seen in 
large objectives. As I had not used a large 
telescope for some eight years, I could not feel 
that my judgment was an entirely critical one ; 
but I am persuaded, that, if any defects exist, 
they are so minute as not to interfere in the 
slightest with the finest performance of the in- 
strument. 
I have been led by the examination above 
described, combined with some experience in 
the use of the Washington telescope, to some 
conclusions respecting the most appropriate 
features in the mounting of an instrument of 
the largest size.. They may here be enumer- 
ated for the consideration of those engaged in 
constructions of this kind. 
1. I think, that, in order to secure the neces- 
sary stiffness with the least weight, the axes 
should be hollow. The material can then be 
made comparatively thin. It is true that the 
greater the friction, the larger the axis; but 
the mass of metal in the interior of the axis 
contributes so little to its stiffness that the ex- 
ternal diameter will have to be increased very 
little to secure the same stiffness with the hol- 
low axis as with the solid one. 
2. It is not worth while to supply the dec- 
lination-axis with friction-rollers, unless exper- 
iment and research shall show that they can be 
made more effective than they appear to be in 
the Vienna instrument. 
3. The best quick motion in right ascension 
is that adopted in the Washington telescope, 
where the observer pulls an endless rope hand 
over hand, and can lock and unlock at pleasure 
the gearing which connects the turning-wheel 
with the telescope. 
4. If, as is probable, the quick motion in 
declination by means of the loose rope attached 
to the two ends of the telescope requires too 
strong a pull, the best method of giving this 
motion is through a gearing turned by an axis 
passing centrally through the polar axis, on 
the Repsold plan; but it is desirable to have 
this motion made by turning a crank, or pulling 
a rope, rather than by taking hold of the wheel. 
5. Coarse divided wheels should be supplied, 
so that the observer, while turning the instru- 
ment, can constantly see its approximate point- 
ing. It is better if this coarse reading can be 
made with the naked eye, as is the case with 
the right-ascension movement in Washington. 
The declination-circle, being farther from the 
observer, has to be read with an opera-glass if 
more than a coarse fraction of a degree is re- 
SCIENCE. 
383 
quired. By such an arrangement the telescope 
can always be set by the quick motion so nearly 
that any object sought shall be in the field of 
view of the finder. In nine cases out of ten 
this will be all that is required in practical use. 
It should never be forgotten that in all quick 
motions it is very desirable that the observer 
should be able to keep his eye upon the move- 
ments of the telescope itself, so to save him 
from even a groundless apprehension that some- 
thing may be going wrong. 
6. The slow motion should, if possible, be 
endless. ‘There is no difficulty in making it so 
in right ascension: there may be, however, in 
declination. 
7. When the instrument is so large that 
there is an interval of three feet or more be- 
tween the centre of the polar axis and the side 
of the tube, the screw which communicates the 
clock-movement should be geared into a com- 
plete circle rather than into a sector. The 
use of a metal band to multiply the interven- 
ing radius of the wheel offers no advantage, in 
the case of large instruments, to compensate for 
the disadvantage of want of stability arising 
from elasticity of the band and its fastenings. 
8. In this connection the question arises of 
applying the Greenwich system, which consists 
in setting the hour-wheel so that its position 
shall correspond to sidereal time, and clamping 
to it a second wheel corresponding to right as- 
cension. Every practised astronomer is famil- 
iar with the trouble in setting an ordinary 
equatorial, arising from the necessity of having 
to calculate the constantly varying hour-angle 
of the object on which he points. With the 
Greenwich arrangement there is no such trou- 
ble. The worm-wheel being once set to side- 
real time, the observer has only to set the other 
one to the constant right ascension of the ob- 
ject. It is true that practical difficulties arise 
in the usual construction, owing to the fact that 
the vernier on the gear-wheel will from time to 
time be on every point of the circle; but this 
difficulty can, I think, be obviated by appropri- 
ate arrangements. 
9. A clock-motion which can be kept up by 
water or other power is greatly preferable to 
any system which requires an assistant to wind 
up a weight. 
10. The entire practicability of illuminating 
the divisions of the circle by lamps, and of 
reading these divisions from the eye-end of the 
telescope, has been so completely demonstrated, 
that all large instruments should be supplied 
with this arrangement. 
11. The system of illuminating wires, field- 
micrometer slit, etc., by a single lamp which 
