DOES IT TAKE TIME TO THINK? 565 



nebrook as Iiaving taken place four-fifths of a second later than the 

 time observed and noted by Maskelyne. All attempts by Mr. Kin- 

 nebrook 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 de- 

 fect 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 de- 

 gree to which the personal element enters into all work, and espe- 

 cially 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 rec- 

 ords 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 

 amoimt of this error was larger with Mr. Kinnebrook than with Mas- 

 kelyne. 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 dif- 

 ference between the amount of error habitual to one of these observ- 

 ers 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 ex- 

 periments it appeared that one who observes and records an occur- 

 rence 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 experi- 

 ment, 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 per- 

 sons for the same class of events. It also differs in the same person 

 for events of different classes. The time required to observe and 



