524 Proceedings of the Royal Irish Academy. 



Talue in 1823. "Wolf proves his assertion by the fact, that the equa- 

 tion Bessel- Argelander, by occultations of stars, was found = — 0''222, 

 but by transits = - P-222. I do not understand this argument, as 

 there is no reason why Bessel should count the seconds properly while 

 observing occultations, but not when he observed transits. 



!N'otwithstanding these but slightly proved objections to Bessel's 

 explanation of his great error, nobody has attempted to deny the great 

 influence of the "superposition" of the two senses on the appearance 

 of the personal error. Faye has tried to elucidate the matter by a 

 comparison.* We might imagine, he says, that the intellect was an 

 eye in the interior of the brain, observing the effects which the im- 

 pressions of the senses make in the fibres of the nerves. If impres- 

 sions of the same nature are made in the same point, this interior eye 

 can easily decide whether they are simiiltaneous or not ; but if it 

 should observe different perceptions, by fibres extending to different 

 portions of the brain, the interior eye would have to move from one 

 portion to another, the time spent in this movement would not be 

 remarked, and perceptions divided by a real interval of time might 

 therefore erroneously be considered as simultaneous. The time lost 

 in passing from one perception to another is different in each indivi- 

 dual, and in this way personal equations may arise. This is only a 

 comparison, but a veiy good and instructive one. 



"We cannot, however, be satisfied with this, but we must examine 

 the question more closely in order to see how a personal error can 

 arise, both in the eye-and-ear method, and in the chronographic one. 

 Let us begin by considering the different effects of the senses, and the 

 time spent in their completion, as this time already gives the possibi- 

 lity of a personal error. There are three processes by which a per- 

 ception is made : an impression on a receptive organ (the eye or the 

 ear), the passing on of this impression through the nerves to the 

 Ibrain, and at last, the mind's perception. 



It is quite clear that there must be some time lost dui-ing the two 

 first processes, and this is besides proved in different ways. The 

 impression on the receptive organ is quite material, and lasts some 

 time; very brilliant objects are, for instance, visible to the eye a 

 short time after this organ has been closed. That the passing on to 

 the brain through the nerves requires time may be seen from direct 

 experiments. Helmholtz has, for instance, found the velocity of the 

 propagation of a nervous irritation equal to about thirty-four metres 

 in a second,! so that the time lost in bringing an impression to the 

 brain is often quite perceptible. 



In like manner, a certain time must elapse before the material 

 irritation causes the mind to be aware of what has happened. As we 

 do not know at all how the perception arises, that space of time can- 

 not be determined, but its existence can indirectly be felt if we deter- 

 mine the velocity of the sensation, or the time in which the mind can 

 only be occupied by a single perception. We may determine this by 



* Comptes rendus, t. lix., p. 475. t Poggendorf 's Annalen, Bd. Ixxix., p. 329. 



