394 THE POPULAR SCIENCE MONTHLY. 



An apparatus similar to this w^s invented and used by Wolf, of the 

 Paris Observatory, and we owe to him much the fullest account of 

 personal equation which we have. We cannot do better than to give 

 a brief abstract of his memoir ("Memoires de l'Observatoir e de 

 Paris," tome viii., p. 153), as the results obtained by the American de- 

 vice have not been made public. 



His first experiences showed him that his absolute personal equa- 

 tion, when he used the chronographic method of recording, was ex- 

 tremely small (from three to four hundredths of one second) ; and, 

 although this was an interesting fact, yet the very smallness of this 

 equation showed that it was hopeless to attempt to discover the laws 

 of variation of so minute a quantity. 



These laws would be masked by the accidental errors : so that all 

 the observations of M. Wolf have been by eye and ear. It should be 

 stated that M. Wolf is an observer of experience. In his own experi- 

 ments he proposed to himself to determine the effect on his equation 



(a.) Of the position of the observer (sitting or standing, etc.). 



(b.) Of the magnifying power of the telescope. 



(c.) Of the direction of motion of the star (i. e., whether from right 

 to left, or the reverse). 



(d.) Of the brightness of the star. 



His personal equation he found was, at first, about +0 S .3; and in 

 a short time this fell to +0M ; this was undoubtedly due to the fact 

 that the observer felt in what direction his observations had to be 



* 



modified, in order to bring them nearer to the truth, and that he un- 

 consciously so modified them. This, however, did not continue with- 

 out limit ; his personal equation remained, for all the time he observed, 

 at this lower limit, and this fact gave him the first clew to the physio- 

 logical explanation of the phenomenon. 



M. Wolf finds that the brilliancy of the star has no sensible effect 

 on personal equation, a conclusion identical with that derived by Mr. 

 Dunkin, of the Royal Observatory at Greenwich (" Monthly Notices, 

 Royal Astronomical Society," vol. xxiv., p. 158). 



With regard to the influence of the direction of motion of the arti- 

 ficial star, M. Wolf finds in his own case a mean constant difference 

 of 8 .04 obtained from over 400 transits : this he subsequently explains 

 by the fact that, if his right eye be fixed on two dots equidistant from a 

 line drawn on a sheet of paper, one of these dots always appears nearer 

 to the line than the other by a small quantity. This, of course, is a 

 defect in the symmetry of the eye, and it is quite a common defect, 

 which probably many of the readers of The Popular Science Month- 

 ly have, perhaps without knowing it. 



The influence of the apparent velocity of the star Bessel states to 

 have been nothing in his own case, provided the star was situated 

 more than 20 from the pole. Wolf's experiments do not agree w T ith 

 this, and he confirms the researches of Dr. Pape and of Dunkin. 



