nebrook as having taken place four-fifths of a second later than the time observed and noted by Maskelyne. All attempts by Mr. Kinnebrook to account for or to remedy what he deemed his fault were in vain. At last Maskelyne, assuming that his own observations were correct, and that the habit of his assistant arose from some defect difficult to explain, felt obliged to discharge him as incompetent for the special class of work in which he had been long engaged. At the present day astronomers, with better knowledge of the degree to which the personal element enters into all work, and especially into a class of labor so difficult as that of recording fractions of a second or hundredths of an inch, are accustomed to place as much reliance upon the observations of Mr. Kinnebrook as upon those made by the royal astronomer himself. The constant difference in the result is explained, not by the assumption of incompetency on the part of one of these astronomers, and of complete accuracy on that of the other, but by reference to what is known as the relative personal equation of the two men. It is not supposed that the records made by either of them are absolutely correct. Beyond a doubt there is in the records of each a small and constant error arising from personal characteristics. There is no reason to suppose that the amount of this error was larger with Mr. Kinnebrook than with Maskelyne. The absolute personal equation, as the error is called, is as likely to appear in the observations of one as in those of the other. The error of four-fifths of a second, which invariably appeared, does not represent a difference from the exact truth, but the constant difference between the amount of error habitual to one of these observers and the amount of error habitual to the other. But, as we do not know the personal equation of either, in this, which is the first recorded case of the kind, we can never know where the truth actually was.
By this and similar cases the attention of scientific men was called to the effect of personal characteristics in classes of work similar to that of the astronomical observatory. Examination showed that these characteristics are a constant cause of error. By numerous experiments it appeared that one who observes and records an occurrence always gives a result which differs from the exact truth. Even where the observer was trained and skilled in observing events like those in the experiment, the rule was the same. He recorded the time too early or too late. The error would appear in each experiment, and always to the same amount. If the record was too late in one, it invariably was too late. This habitual difference between the time as noted by an observer and the actual time of the happening of the occurrence is what has been termed the absolute personal equation of that observer. It represents the amount of error which he will always make. It has been found to differ with different persons for the same class of events. It also differs in the same person for events of different classes. The time required to observe and
