SOURCES OF THE NEW PSYCHOLOGY. 99 



those of another observer for the same star. They found a differ- 

 ence of half a second. Later he made similar experiments with 

 Argelander and Struve, with the result of always finding a per- 

 sonal difference. 



Bessel sought for the cause of this "personal equation" by 

 varying the conditions. He first made use of the sudden disap- 

 pearance or reappearance of a star instead of steady motion. The 

 personal difference was much decreased. This seemed to indicate 

 that the trouble lay in comparing the steady progress of the star 

 with the sudden beat of the clock. The next step was to change 

 the beats, with the result that for Bessel the observations were 

 made later with the clock beating half seconds than with one 

 beating seconds, whereas Argelander and Struve showed no par- 

 ticular change. One other point was investigated namely, the 

 effect of the apparent rate of the star ; within wide limits the per- 

 sonal equation was not changed. 



About 1838 the personal equation began to receive regular 

 notice in astronomical observations, as appears in the publica- 

 tions of Airy * and Gerling of that year.f 



It was natural to wish for a comparison of the astronomer's 

 record with the real time of transit. At the suggestion of Gauss, 

 an artificial transit was arranged by Gerling, the object observed 

 being a slow pendulum. This is probably the first measurement 

 of a reaction time. In 1854 Prazmowski J suggested an apparatus 

 carrying a luminous point for a star and closing an electric cir- 

 cuit at the instant it passed the line ; a comparison of the true 

 time with the astronomer's record would give the real amount of 

 his personal equation. From this time onward various forms of 

 apparatus were invented and numerous investigations were car- 

 ried out. The astronomers found that in such observations 

 sometimes the star was seen to pass the line too soon, sometimes 

 too late, and that the error varied with every variation in the 

 method of observing and in the mental condition of the observer.* 



Let us turn for a moment to another science. The new physi- 

 ology, begun by the pupils of Johannes Muller, in which the 

 phenomena of life were to be explained by physical and chemical 

 processes, had undergone a remarkable development. Du Bois- 

 Reymond had taught how to apply the experimental methods 

 and apparatus of physics to the study of physiological processes. 

 Soon after this Helmholtz measured the velocity of nervous 

 transmission (1850), an experiment that Johannes Muller had 



* Greenwich Astron. Observations, 1838, p. xiii. 

 f Astron. Nachrichten, 1838, vol. xv, p. 249. 



\ Comptes rendus, 1854, vol. xxxviii, p. 748. 



* For the history of the personal equation, see Sanford, Personal Equation, Am. Jour. 

 Psych., 1888, vol. ii, pp. 3, 271, 403. 



