DETERMINATION OF PRECISE TIME—SPENCER JONES 193 
more than two or three hundredths of a second; it is for this reason 
that this form of micrometer is called the “impersonal”? micrometer. 
Small though these residual personal equations are, they remain 
remarkably constant and can be determined by personal equation 
machines. They seem to arise from two causes: there is “bisection 
error,” an observer systematically bisecting an image to the right or 
to the left of its center; this error changes sign at the zenith with 
instruments in which the observer changes the direction in which he 
faces, according to whether he is observing a north or a south star; 
there is also “‘following error,” an observer systematically setting the 
wire in front of or behind the center of a moving image. This error 
does not change sign at the zenith. 
If the pivots are not exactly cylindrical, the telescope will be 
twisted out of the meridian by an amount varying with its position. 
The figures of the pivots must therefore be determined with great 
accuracy and appropriate corrections applied to the observed times of 
transit. The figures of the pivots must be determined at intervals, 
as they may change slowly in the course of use through wear. Other 
variable errors can be introduced through slight mechanical imperfec- 
tions in the telescope; if there is the slightest play in the eyepiece 
micrometer or in the objective, errors will be introduced which will 
vary with the position of the telescope. 
When all the possible sources of error which can affect observations 
with a transit instrument are borne in mind, it is rather surprising 
that the observations are as accurate as they are. The probable error 
of a single time determination is usually about two-hundredths of a 
second. This was quite accurate enough before the era of clocks of 
high precision and before there were any practical requirements for 
very precise time. The scatter of the observations is, however, incon- 
veniently large for the adequate control of the performance of the 
modern quartz-crystal clock. For these reasons the conventional 
transit instrument is likely to be gradually replaced for the purpose 
of time determination by some other type of instrument. Several 
modifications of the transit instrument have been considered which 
eliminate or minimize some of its disadvantages. The most accurate 
results are given, however, by an entirely different instrument known 
as the photographic zenith tube. It consists of a fixed vertical telescope 
pointing to the zenith, which has a mercury horizon at the bottom of 
its tube, whose purpose is to reflect the light from a star to a focus in 
the plane of the second principal point of the objective. The funda- 
mental principle of the instrument was due to Sir George Airy, who 
first used it for the reflex zenith tube at Greenwich: when the light is 
brought to a focus accurately in the plane of the second principal 
point of the objective, the results are unaffected by tilt of the telescope. 
The troublesome error of level is therefore immaterial, while any error 
