The morning call., 1893-12-02


through it for the illumination OF the mi
cromater and other threads. The vertical
axis Is supported by column about 81
centlmeters GA inches) in height and at Its
lower end carries clamp and vernier IN
connection with the aZimuth circle. The
horizontal circle is about 80 centimeters
az inches) in diameter, and Is graduated
to read to half-minutes or less; two NOV
able stops can be applied to it. defining on
the instrument the position OF the plane
OF the meridian and without interfering
with the motion of the axis while revers-
Ing. The whole rests upon three foot
screws. by means of which the instrument
can be leveled.
A delicate level (value of one division
about three-fourths of a second) at right
angles to the horizontal axis is connected
with the telescope, and reyolves on cen
ter so as to Indicate the inclination OF the
telescope; concentric with the level pivot
and firmly connected with the tube Is a
graduated 30-centimeter Q2-lnch) SEMI
circle, or a 15-centimeter (6-inch) full cir
Gle. on which zenith distances are read of
10 the nearest 80 sec. by means OF the arm
and vernIer attached to the movable level.
The telescope can be set to any inclination
and elamped; for accurate pointing. the
bubble is brought into the middle by a
hne-motiOn screw.
The micrometer screw carries movable
zenith distances; its head is divided in 100
parts. of which tentbs may be estlma ted ;
the whole numb of turns are read OF by
means of rack. shown on the side OF the
field Of view. The value Of one revolution
OF the n:icrOmeter is about 10 sec. There
are also two fixed threads parallel 70 the
micrometer thread, about 10 min. or 90
min. apart, indicating the range between
which the latter is ordinarily employed.
10 provide for the case of transit observa-
tons, there are also five equldtstant vertl-
cal threads Inserted symmetrlcaily over the
optical center of the field. For con-
venience OF observing, the telescope is
supplied with prismatlc eye-plece. Inc
Instrument may be mounted on blocks OF
stone. brick or WOOD.
Some idea of the fineness OF the work
may be gathered from the following ''di-
rections for Observing:
The instrument being adjusted and the line
OF collimaton OF the telescope placed in the
meridian in which position the aZlmutbai mo-
ton Is arrested by tie stops, the index OF the
vertical circle is set TO the mean zenith distance
OF the first pair taken from the list previously
prepared, on which list The chrOnometer time
OF elimination of each star for the night is
noted. The telescope is then directed 10 that
side OF the zenith where the first star Will cul-
mlnat. and The bubble OF the level is made TO
play very nearly in the middle. AS soon as the
STAR enters the field, and when transiting on
one OF the vertical wires, or at any convenient
DumDef OF seconds before the cultntnatlu1, the
observer viii pick up the beat OF the chronome-
Ter ana bisect the star with the micrometer
thread and keeping ii bIsected up 10 the instant
of elimination; the level and mcrometer are
then read, the instrument is revoyed 80 deg.
and the second star is observed IN The same
manner. During these observations tue tan
gent screw OF The vertical circle must not be
touched, though the tangent screw which gives
a slow motion 10 the telescope (and conse-
Quently also 10 the leveD may be used after the
reversal Of the instrument In the exceptional
case when the vertical axis Iof The instrument
is not well adjusted.
II for some reasons the meridian observation
falls, the star may be bisecled of the meridian
and the lime noted. This may be done in two
ways, either by moving the telescopa in aZimuth
and bisecting in the line of colImatiou or by
observing the star off the middle of the field
leaving the telescope undisturbed in the plane
OF The meridian. The latter method Is generally
the preferable one, particularly when the star
eliminates near the zenith. If, however. the
inef1ulonal distance OF the star be considerable,
the first method had better be followed.
The directions conclude indulgently that
leach pair of stars is generally observed
on five or six nights; greater number Of
observations would add but very little to
the value of the mean result"
Professor Davidson is trying 10 get the
necessary observations, but is at preseB t
handicapped by the obscure weather. The
result of his labors will be awaited with
interest.