Page:Popular Science Monthly Volume 6.djvu/410

 An apparatus similar to this was 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 ("Mémoires de l'Observatoire de Paris," tome viii., p. 153), as the results obtained by the American device have not been made public.

His first experiences showed him that his absolute personal equation, when he used the chronographic method of recording, was extremely 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 experiments he proposed to himself to determine the effect on his equation—

His personal equation he found was, at first, about +08.3; and in a short time this fell to +08.1; 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 unconsciously so modified them. This, however, did not continue without 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 physiological 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 artificial star, M. Wolf finds in his own case a mean constant difference of 08.04 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 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 with this, and he confirms the researches of Dr. Pape and of Dunkin.