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TEE POPULAR SCIENCE MONTELY.— SUPPLEMENT. 



and the human mind struggles amid this ocean of 

 errors. 



VI. 



Astronomers long ago remarked that the same 

 luminous sensation, striking the eye of two ob- 

 servers, is not caught at exactly the same instant 

 by both. They are observing the moment when 

 a satellite is about to disappear behind Jupiter ; 

 whatever care they take, they will not mark the 

 contact at precisely the same instant, and, if they 

 try again, the variation in their observations will 

 remain the same : one of the two astronomers, 

 do what he may, will always be a little behind 

 or a little ahead of the other by about the same 

 fraction of a second. As it could not be sup- 

 posed that the light took different times to pass 

 through the glass or the eye of each observer, 

 there was no other way than to refer to a differ- 

 ence in the rapidity of nervous action those per- 

 sonal errors for which allowance is made in as- 

 tronomical calculations. It is natural enough, 

 ■when we think of it, that functions closely con- 

 nected with the material conditions of an organ, 

 even though it be the brain, present, as the organ 

 itself does, appreciable differences in different 

 persons. There was but a step from this to 

 measuring the time taken by various acts, even 

 those of the interior sense. A physiologist of 

 Utrecht, Donders, lately undertook the construc- 

 tion of two instruments, as delicate as ingenious : 

 one intended, to use his own words, "to measure 

 the time taken by certain mental operations ; " 

 the other, " to measure the minimum time required 

 for the production of an idea." Donders gave 

 his instruments barbarous names, as are most 

 of those manufactured out of the most harmo- 

 nious language in the world : he called one a 

 nematochograph, and the other a nematochomcter. 

 The first is merely a registering apparatus ad- 

 justed to the measurement of exceedingly minute 

 fractions of duration : a clock-work movement 

 gives rapid rotation to a cylinder covered with 

 lampblack ; a bit of feather, fixed on the end of 

 a tuning-fork which is set vibrating, tracing a 

 wavy line on the moving cylinder. The note of 

 the tuning-fork gives the number of undulations 

 in a second ; every undulation consequently rep- 

 resents a corresponding fraction of a second ; 

 and thus four-hundredths or five-hundredths of a 

 second are successfully measured. Now, if we 

 wish to know the time the brain takes to per- 

 ceive an impression made on one of our senses 

 by a prick, the light of a spark, or a brief sound, 

 no matter which, the instrument is arranged so 

 that the phenomenon which affects the touch, 



the eye, or the ear, instantly records itself on the 

 blackened cylinder alongside the wavy line in- 

 scribed by the tuning-fork. The person making 

 the experiment, as soon as the impression is felt, 

 must touch lightly with his finger a spring that 

 makes a second mark on the turning cylinder. 

 The number of undulations that separate this 

 mark from the first one indicates the fraction of 

 a second that has gone by — that is to say, the 

 time needed for the impression to go forward, to 

 become a conscious perception, and to stimulate 

 the act of volition conveyed in its turn to the 

 muscles. Now, the latter part of the circuit, 

 after the act of volition, remaining always con- 

 stant, we understand how, by varying his experi- 

 ments, Donders succeeded in reaching the dis- 

 covery whether a luminous sensation is perceived 

 more quickly than an acoustic or a tactile sensa- 

 tion. 



The nemaiochograph, then, in this case, meas- 

 ures a very complicated operation ; but it is not 

 the same with the next experiment : instead of a 

 simple sensation which only needs to have its 

 instant of perception well marked by its subject, 

 the question here is, how to resolve a dilemma. 

 The subject of the experiment is placed in the 

 dark, and a light, either green or red, flashed on 

 him, and the answering signal given by the right 

 or the left hand according to the case. The 

 whole of these mental operations, it is true, take 

 much more time ; but, as all the elements of the 

 former experiment exist in this one, the calcula- 

 tion of the time taken by the former suffices to 

 give the time occupied by the brain in deciding 

 that the light was red and not green, and that 

 one hand and not the other must act. This Don- 

 ders calls " the time required for the psychical 

 act of drawing a distinction." 



The other instrument, the nematochomcter, is 

 intended for a still more subtile analysis, if that 

 be possible, of mental operations. It serves, to 

 use the inventor's expression, "to measure the 

 time of a simple thought." A simple thought, 

 for instance, would be such a one as this : Two 

 sensations, one acoustic, the other luminous, will 

 reach the brain nearly at the same time ; which 

 will have preceded the other? The instrument 

 is constructed on a different principle from the 

 first: a weight falls on a bell, and at the same 

 time produces a spark. The interval between the 

 sound and the light, though infinitely short, yet 

 must always be determined with extreme preci- 

 sion ; and there must also be the power of making 

 it vary. The instrument being thus regulated, 

 the endeavor is to find what time must be made 



