372 THE NERVOUS SYSTEM. 



tude with which they perceive and record sensible phenomena. This 

 was first noticed in astronomical observatories, where it was found 

 that the passage of a star across the thread of a transit instrument was 

 differently recorded by different observers ; the difference sometimes 

 amounting to as much as one second. Subsequent observations 

 showed that in no case was the time recorded with absolute accu- 

 racy; but that a certain delay always intervened, due to the time 

 occupied by the nervous mechanism of the observer. This fact was 

 established by imitating the transit of a star by means of a luminous 

 point moving with uniform velocity before the field of a telescope. By 

 contrivances like those above described, the real instant of the passage 

 of the luminous point across the thread of the telescope is recorded 

 upon a revolving cylinder, and the observer also marks its passage by 

 similar means. The difference between the real and the observed time 

 represents the " personal error" of the observer. As it is important to 

 eliminate this error from the record of astronomical observations, when 

 its amount has been determined for any particular observer, his record 

 is corrected by a corresponding quantity. This is termed the "personal 

 equation " of the observer. 



The error of any particular individual remains nearly the same, as 

 compared with that of other persons; that is, it will be habitually 

 greater or less in one observer than in another. But, like all physio- 

 logical peculiarities, it varies somewhat in the same individual ; and 

 according to Kampf,* it may change perceptibly even in one night. 

 The following table shows the varying personal equation of two differ- 

 ent observers in fractions of a second, as ascertained on three successive 

 days : 



ABSOLUTE PEKSOXAL EQUATION OF 



Lieut. Tillman. Dr. Kampf. 



May 1 0.125 . . . 0.027 



May 2 0.121 . . . 0.021 



May 3 0.116 . . . 0.026 



Where extreme accuracy is required, as in observations for longitude, 

 it is consequently recommended by Dr. Kampf that the personal error 

 of the observer be determined and corrected at the time of each obser- 

 vation. 



* On the Determination of Personal Equations. Report upon United States Geo- 

 logical Surveys west of the One Hundredth Meridian. Washington, 1877, p. 475. 



