228 PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



«vliicli corresponds to a velocity of twenty-six meters per second. This 

 velocity, however, varies somewhat with the conditions of the organ on 

 which the experiment is performed. I shall not dwell on these points, 

 which are already pnblished, but shall call your attention to this re- 

 markable fact, bearing directly upon our thesis, namely, that the meas- 

 urement of the nervous transmission enables us to rise to the study 

 of psychical action. " Has thought no longer the infinite rapidity 

 Avhich has been habitually attributed to it, and is it possible to measure 

 the time necessary for the formation of an idea or a determination of 

 the will ? " Such are the terms in which Bonders expresses the prob- 

 lem. 



The first researches on this interesting subject are due to the astrono- 

 mers. Toward the year 1790 a curious fact was announced by Maske- 

 lyne, who stated that in the estimation of the passage of stars across 

 the thread of a meridian telescope, there was a constant discrepancy 

 between his observations and those of his aid, Kinnebrock. Later, 

 Bessel, comparing the observations of other astronomers with his own, 

 l^erceived that most of the observers signalized the passage of stars a 

 little later than he did, the difference in some cases being more than 

 a second. These observations attracted the attention of astronomers 

 generally. They commenced to study the jihenomenon under the name 

 of the personal equation^ and to ascertain its absolute value. To illustrate 

 the method of obtaining this result, we shall only describe the process 

 lately invented by M. Wolf, of the Paris observatory. He arranged a 

 luminous bull's-eye, a kind of artificial star, so that it moved at a uni- 

 form rate along a curved line, resembling the trajectory of a true star. 

 At the moment this luminary actually passes before the thread of the 

 telescope, or at the moment when its center coincides with the central 

 thread, it closes the circuit of a galvanic battery ; at this precise instant, 

 an electric current, excited by the completion of the circuit, records the 

 passage of the star on a revolving cylinder. Moreover, the observer, at 

 the moment when he perceives the passage of the artificial star before the 

 thread of the telescope, by jiressing a spring, records the instant on the 

 same chronograph. The interval between the two signals, estimated in 

 fractions of a second, measures the lapse of time between the real pas- 

 sage of the star and the estimation of its passage by the observer. This 

 is the absolute value of the personal equation, which remains nearly the 

 same for each observer, unless, being aware of it, he endeavors to cor- 

 rect it. M. Wolf reduced his error from three-tenths to one-tenth of a 

 second. 



From a f)hysiological point of view, we may ask. What is this personal 

 equation 1 The astronomers, Bessel and Faye, have suggested the hy- 

 pothesis that an intellectual operation is necessary for transmitting by 

 a signal a perceived sensation. The phenomenon has not the less gen- 

 erality from having been at first signalized by astronomers, and it may 

 be said that a certain time always elapses between the instant of occur- 

 rence of a phenomenon and the signal of an attentive observer that he 

 perceives it. The duration which separates the impression from the 

 signal of the reaction has been called t\i& 'physiological period. We owe 

 very curious observations upon the variations of this physiological 

 period to M. Hirsch, (of ISTeufchatel,) and especially to M. Bonders, and 

 his pupils. 



Thus, the signal of reaction being always the same — for example, a 

 motion of the hand, as in closing a galvanic current — it is observed that 

 the signal is produced more rapidly if the impression is made upon the 

 sense of hearing than if it is exercised uijon the sense of sight, and still 



