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POPULAR SCIENCE REVIEW. 
The rays from the star, after passing through the object-glass, 
are received upon the mercury and reflected up through the 
glass again, forming a focal image just above it. In the focal 
plane are the wires of a micrometer by which the star’s image 
is twice bisected, the object-glass being reversed between the 
bisections. A measure of the star’s zenith distance is thus 
obtained which is remarkably free from instrumental errors. 
During the past two years a zenithal telescope of 3-feet 
focus, filled with water, has been used upon the same star to 
determine whether the aberration is altered in amount by the 
passage of the star’s light through a considerable thickness of 
a refracting medium — a question which has been raised by 
some continental astronomers. The observations hitherto have 
negatived the anticipated alteration. 
Equatorially-mounted telescopes have always been regarded 
as of secondary importance at Greenwich. When Sir George Airy 
entered office he found the equipment in this department, to 
use his own word, 44 contemptible.” The best equatorial then 
was one presented by Sir G. Shuckburgh, made by Ramsden, 
with a 4-inch object-glass, a shaky mounting, and in a very bad 
situation. In 1837 a 6f-inch glass was presented to the Ob- 
servatory by the late Rev. R. Sheepshanks ; it was a somewhat 
inferior one, and it was mounted with circles too small for 
accurate measurements. So about fifteen years ago Sir George 
Airy planned the Great Equatorial, deeming it essential that the 
Observatory should have a first-rate instrument of the class, 
even if it was for a time regarded as a luxury. The instru- 
ment (without the covering dome) is figured in the annexed cut. 
Its object-glass, by Merz, of Munich, is of 12f-inch aperture, 
and 17 feet 6 inches focus. The mounting is of the English form, 
which admits of large circles, and such were regarded as essen- 
tial ; the hour circle is 6 feet diameter, that for Declination 
4 feet. The polar frame is formed of iron columns firmly 
braced, and connected at top and bottom by oval iron frames. 
Except the tube, which is of wood, every large part is of iron, 
and the whole was designed with the highest engineering skill. 
Gas is carried to all the micrometers about the instrument, and 
to the eye-piece ; and galvanic wires are led to a chronographic 
touch-piece at the eye-end and to a chronometer there also, 
which is always in front of the observer at the telescope, and is 
controlled by the Sidereal Motor Clock. An observing chair, 
moving up and down upon a travelling frame, enables the 
observer to command all positions without leaving his seat. 
The driving-clock is mounted in a lower apartment ; it consists 
of a water turbine controlled by a conical pendulum. 
We have spoken hitherto solely of the tools of the Greenwich 
