192 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
have to be determined with great accuracy and appropriate corrections 
made to the observations. Flexure of the axis can cause troublesome 
systematic errors. If the horizontal axis is not equally stiff in all 
directions, its flexure will vary according to the direction in which the 
telescope is pointed. If the two halves are not equally stiff, the 
telescope will be twisted from the meridian by a variable amount. 
Personal equations between different observers are somewhat trouble- 
some, though they do not exceed a few hundredths of a second when 
the so-called impersonal micrometer is used. Before its introduction, 
the method of observing was for the observer to press a hand-tapper 
at the instant the star crossed each of a number of vertical spider 
wires in the focal plane of the telescope; by so doing, he closed an 
electric circuit which sent a current to a recording chronograph, which 
recorded not only the signals from the telescope but also time signals, 
every second or alternate seconds, from the clock. The instants 
of the star crossing the wires could then be read off at leisure after the 
observations had been completed. With this method of observing, 
the times determined by different observers could differ by as much as 
halfasecond. The reason is easy to see; one observer might wait until 
he saw the star actually bisected by the wire before he pressed the 
tapper, with the result that, because of the time required for the mes- 
sage to travel from his brain to his eye and to be converted into 
muscular action, his signal would inevitably be late; another observer 
would, as it were, shoot the flying bird, gauging the rate of motion of 
the star so that his tap is made as nearly as possible at the instant at 
which the star is actually bisected. The personal equations can be 
determined by what are called personal equation machines; the 
transit of an artificial star is observed, the times at which the star is at 
certain positions during the transit beng compared with the observed 
times. Although an observer will unconsciously form a fixed habit 
in observing so that his personal equation remains substantially 
constant, small variations, depending upon the physical condition of 
the observer, do occur. 
The method of observing now almost universally employed is to 
have a single movable wire in the micrometer eyepiece instead of 
a number of fixed wires. The wire can be traveled along by the 
observer, who adjusts its speed so as to keep the star continually 
bisected by the wire. As the wire moves along, contacts are auto- 
matically made in certain positions, sending signals which are re- 
corded on the chronograph. In order to relieve the observer of some 
of the strain of maintaining a uniform motion of the wire, it is now 
common to drive the wire mechanically at the speed appropriate to the 
motion of the star, using an electric motor with some form of con- 
tinuously variable gearing. With this method of observing, the 
personal equations of different observers are very small, usually not 
