Dreyer — On Astronomical Transit Observations. 525 



■seeking the mininnim of time in -which two different efforts of the 

 mind can follow each other. We could, for instance, find the time 

 which elapses between the observation of an instantaneous sound, or 

 of a glimpse of light, or the touch of an external object, and the 

 immediate completing of a galvanic current through the pressure 

 of a key by the hand. Experiments to this effect have been made by 

 Hirsch, Hankel, and others. On an average we can consider the velo- 

 city of the perception of a sound, of a glimpse of light, or of the sense 

 of feeling = 0'*20. This number contains the velocity of the propa- 

 gation in the nerves, and, besides, the time which elapses between the 

 arrival of the nervous irritation from the brain to the muscles of the 

 fingers, and the contraction of these.* But as these intervals are very 

 small, compared with the above numbers, they prove with certainty that 

 a limitedtime is necessary for the accomplishment of the mental process. f 



The above-mentioned experiments show that the velocity of the 

 sensation contributes to the formation of a personality in the percep- 

 tion of a phenomenon, as the single observers did not find exactly the 

 same results. In the chronographic method, where the eye and the 

 hand work together, a different estimation of the coincidence of the 

 star and the wire can be joined by a different way of pressing the 

 key. The former circumstance seems to be of great importance, 

 especially if we observe bright stars with instruments of small aper- 

 ture, which often do not give sharply-defined images of the stars. In 

 the eye-and-ear method the different velocity of perception may be 

 joined by the " superposition" of the two active senses, as well as by 

 a different manner of estimating the beats of the clock, perhaps, also, 

 by the expectation of them ; while the chronographic method only 

 requires the action of two of the senses, the eye-and-ear method takes 

 in reality four actions of the mind. One might conclude from this 

 that personal errors in the chronographic method, within shorter 

 intervals of time, change less than in the eye-and-ear method, and that 

 greater variations do not appear as suddenly in the former as in the 

 latter, in which greater variations may be expected even within 

 shorter intervals. As we have seen in the foregoing pages, obser- 

 vations have in part confirmed this conclusion, and it is a fact that 

 errors of such an extent as Bessel's, and some of the observers, 

 mentioned in the note of page 523, have never been found in the 

 chronographic method, which certainly proves the great influence the 

 simultaneous working of the senses and the expectation of the beats 

 of the lock have on our estimation of a transit. 



A general theory cannot be given of the origin of the personal 

 error. "We can only point out different circumstances which contribute 

 to the formation of a personal error, and there can be no doubt that 



* By Helmholtz found = O^-Ol. 



t If it is necessary to prove more fully the dui-ation of an impression on the 

 senses, we need only remember that glimpses of light or sounds which follow 

 one another with shorter intervals than about O^^-OS, cannot be perceived as separate 

 phenomena. 



3 G 2 



