Dreyer — On Astronomical Transit Observations. 485 



folded round a slowly revolving cylinder, wliile the observer himself, 

 in the moment he sees the star bisected by the wire, establishes (or in 

 some apparatuses, interrupts) a current with a key, and in this way 

 makes a mark on the paper, between two second-marks. He can then, 

 afterwards, measure the distance between these marks, and determine 

 the fraction of the second with great exactitude. In both methods of 

 observing, it has been found that there exists a difference between the 

 moments of culmination of a star, as found by different observers, 

 using the same instrument. 



It is to Bessel that we owe the discovery of this personal differ- 

 ence. He was not, however, the first who remarked a different 

 estimation of transits, but his researches on this subject were- 

 occasioned by his finding in the Grreenwich observations from 1795, 

 that one of Maskelyne's assistants, Mr. Xinnebrook, had got into 

 the habit of observing transits over the wires of the transit instru- 

 ment 0''5 to O^'S later than Maskelyne himself. In 1794 and the 

 beginning of 1795, the observations of the two astronomers had agreed ; 

 but in August, 1795, Kinnebrook began to observe half a second later,^ 

 which difference, in 1796, rose to 0^-8. As it was Maskelyne's opinion 

 that his assistant did not use the above mentioned way of observing 

 by eye and ear, but some other irregular method of his own, he dis- 

 missed this, in other respects, skilful man. The matter was looked 

 upon in this way by everybody; no one thought that there had been 

 found a physiological phenomenon, which was perfectly independent 

 of the observer's will.'^' 



Bessel examined the matter again, and showed by his excellent 

 investigations, which in 1823 were published in the eighth section of 

 the Konigsberg Observations, that most observers have a different way 

 of estimating transits. He studied the equations between liimself, 

 Walbeck, and Argelander, and communicated the results in extejiso, 

 together with researches on the variations of the equations from time 

 to time, the influence of the magnifying power, and other circum- 

 stances. With his usual acuteness, he gives, besides, several hints 

 respecting the origin of the phenomenon. The remarkable result of 

 Bessel's investigations was, that he himself observed about a whole 

 second earlier than the two other astronomers. He found : — 



In 1820, Bessel -Walbeck = - p-04. 

 In 1823, Bessel - Argelander = - 1 -22.1 



* Compare the history of the Greenwich Obsei-vatory in vol. ii. of Lincle- 

 nau's and Bohnenberger's "Zeitschrift fiir Astronomie " (1816). 



t Everywhere in this paper a difference A — B = + — is to be understood in 



10 



1 • 1 » 1 '^^ ( later ) 



this way, that A observes — < eadier ( *'"^''^ -"• 



