194. ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
of azimuth does not affect observations made in the zenith. The 
telescope is constructed so that the objective and the plate holder can 
be rotated through 180°, the observations being made photographically 
in order to eliminate personal equations and to give greater accuracy. 
Suppose two exposures are given on a star at times which are sym- 
metrical about the time of meridian transit, the objective and the 
photographic plate being rotated through 180° between them. The 
two images will lie on a line exactly parallel to the meridian. If, 
however, the two times of exposure are not exactly symmetrical, the 
images will be slightly staggered; by measuring the staggering and 
knowing the clock times of the two exposures, the clock time of 
meridian transit can be inferred. 
In practice an exposure of finite length is required to give a 
measurable image on the plate. During this exposure the plate holder 
is traveled along at the speed appropriate to the motion of the star, 
signals being sent to the chronograph at certain definite positions of 
the plate holder. After reversal the plate carriage retraces its path, 
and signals are sent during the course of the second exposure at the 
same positions. 
With this design of instrument, collimation error does not enter, 
there are no pivot errors to be considered, and the various sources of 
error inherent in a movable instrument are avoided. At the Naval 
Observatory, Washington, a photographic zenith tube, designed and 
used by F. E. Ross originally for the determination of the variations 
of latitude, has been used for some years for the determination of time. 
An instrument on the same general principle, but differing materially 
in details of design, is in an advanced stage of construction for the 
Royal Greenwich Observatory. The errors of time determination 
should not exceed 2 or 3 milliseconds, which will permit a tight control 
of the performance of the observatory clocks. 
For the purpose of time determination it is necessary to assume 
positions for the stars which are observed. These positions will have 
random errors, whose effects can be reduced by observing sufficient 
stars. But they may also be affected by systematic errors; if, for 
instance, the errors vary with right ascension they will introduce a 
spurious systematic variation in the derived clock error through the 
year. For the purpose of time determinations and in order that the 
times determined at different observatories can be directly compared, 
there is an international agreement to use the positions of the stars 
given in the fundamental star catalog known as the FK3. These 
are bright stars, whereas with the photographic zenith tube, inasmuch 
as observations are restricted to a narrow belt at the zenith, it is 
necessary to use fainter stars. Their positions must therefore be 
determined by transit circle observations and tied on to the FK3 
system, The photographic observations will in course of time provide 
