526 Proceedings of the Boyal Irish Academy. 



the cause of one ohscrver's error is not the same as the cause of 

 another observer's. Wolf tries, in the paper we have so often quoted, 

 to give a general theory of the personal error, which, however, does 

 not include the very large ones, with respect to which he contents 

 himself with Bessel's explanation, simply adding, that constant 

 practice may diminish them. He recounts so many ingenious ex- 

 periments that this is sufficient reason for us to examine hi& 

 ideas more closely, especially as several of his experiments show 

 that the causes we have mentioned on the foregoing pages are 

 certainly not the only ones which give rise to the personal faults. 



"We have already seen that "VYolf's error in the commencement was 

 found = + 0''3, and that it was afterwards brought down by-and-bye 

 to O'-l, at which value it remained constant. In order to see whether 

 this small correction arose from the confusion of hearing and seeing at 

 the same time, he placed before the telescope a Geissler's tube, which 

 gives a glimpse every second, and in this manner marked the time. 

 One evening he found by common eye-and-ear observation of eighty 

 angle transits, the error = + O'*"!!, and by a series of observations with 

 the tube, instead of the clock (intermixed with the former) the error 

 = O^'IO. Another time the seconds were marked by the star itself, 

 which, as an electric spark, blazed up every second, so that a sudden 

 flash showed the position of the star in the field at the beginning of 

 each second. By stopping his ears, in order to avoid hearing the noise 

 of the spark, the error was found = + O^'OS, while common observa- 

 tions at the same time gave + O'-IO. Accordingly, "Wolf's error was 

 always constant, whether the observation was made by sight alone, or 

 with the assistance of the ear. However, he tried in another way, 

 whether "le temps mort" existed, as if so, its duration would depend 

 on which organ was used. He substituted feeling for hearing by 

 receiving light electric shocks in the left hand with a second's interval. 

 Eighty transits of this kind gave his correction = + 0^- 1 1 exactly the 

 same as common observations:* 



Having convinced himself that the simultaneous working of two 

 senses could not give rise to his personal error, as it remained the 

 same, whether one sense was working or the sight and the feeling^ 

 co-operating, instead of the sight and hearing, "Wolf produced by 

 means of holes in a moveable board three artificial stars, situated one 

 above the other, in a line perpendicular to the direction of their 

 motion. The central star was always visible, the upper and lower 

 one at the same time, but only for an instant, for example, regularly 

 with a second's interval. There appeared, then, a remarkable circum- 

 stance, that at the moment the two outer stars flashed up the observer 

 imagined he saw the central one had moved a little in advance of the 

 other Wo, and this advance seemed proportional with the velocity. 

 However, the foremost star always seemed to be furnished with a ray 

 of light behind (reaching to the spot it should have occupied between 



* Annales do F Obscrvatoiro do Paris, viii., p. 189. 



