RECENT PROGRESS IN GEODESY. 
7 
width of the observing slit, puts the instrument virtually in 
the open air during observations. 
The instruments used at the principal stations are specially 
designed zenith telescopes, by Wanschaff, of Berlin. The 
telescope has a focal length of about 51 inches, an aper¬ 
ture of 4£ inches, and a magnifying power of 104. Aside 
from other minor peculiarities, two are especially noticeable. 
The barrel of the telescope proper is protected by an outer 
thin metallic tube, which is connected at but few points with 
the telescope proper, and serves merely to protect it against 
sudden changes of temperature. This false tube is pierced 
at various points to permit circulation of air in the space 
between it and the tube proper. The eye-piece is furnished 
with a reversing prism of peculiar construction, such that 
all observations may be made with the observing line ap¬ 
parently vertical and with the star apparently moving either 
upward or downward at the will of the observer. The ob¬ 
servation upon one star consists of four pointings, two taken 
while the star is moving apparently upward in the field of 
view and two with the star moving apparently downward, 
the reversing prism being turned 180 degrees between these 
observations. If, then, the observer has a personal tendency 
to place the observing line too far to the right, this will have 
contrary effects in the two pairs of bisections, and the per¬ 
sonal equation will be eliminated from the mean result. In 
so far as accidental errors are concerned, the few observa¬ 
tions already made indicate a high degree of accuracy, the 
probable error of a single observation of a pair being about 
zh 0".10. Few series of observations yet made can show 
probable errors as small as ± 0".16. 
The computed motion of the pole will be nearly independ¬ 
ent of the errors in the assigned declinations of the stars, 
the effects of the errors of declinations being eliminated by 
the well-known group method. The stars to be observed 
are divided into twelve groups, and each group is observed 
for about two months (50 to 80 days, according to the time of 
year). During the first half of the period when group No. 2 
