744 THE POPULAR SCIENCE MONTHLY. 



association, and effort, and is to a great extent a matter of inborn 

 capacity. The physicists express this by saying that each of us 

 has his personal equation. Perhaps this will be more easily under- 

 stood if we follow the manner in which it was discovered. One 

 of the interesting astronomical events is the eclipse of Jupiter's 

 moons as they pass behind the planet and disappear from the 

 astronomer's view. Maskelyne, British astronomer royal, and his 

 assistant in the Greenwich Observatory, in 1795, sitting side by 

 side and looking through two telescopes, were attempting to re- 

 cord very accurately the moment at which the eclipse was com- 

 plete. It was found that their records differed from one another 

 by some fractions of a second. And the differences were about 

 the same when other observations with a similar object were 

 made. The explanation of these differences has been found, after 

 many years of investigation, to be due to a difference in the rapid- 

 ity with which each man observed and recorded his observation, 

 and those differences can now be measured. This was not appre- 

 ciated at first, for we find that the result of this discovery of a 

 difference between the records of the two observers was very un- 

 fortunate to one of them; for in his annual report Maskelyne 

 writes : 



" I think it necessary to mention that my assistant, Mr. David 

 Kinnebrook, who had observed the transits of stars and planets 

 very well in agreement with me all the year 1794, and for the 

 great part of the present year, began from the beginning of Au- 

 gust last to set them down half a second of time later than he 

 should do according to my observations ; and in January of the 

 succeeding year, 1796, he increased his error to eight tenths of a 

 second. As he had unfortunately continued a considerable time 

 in this error before I noticed it, and did not seem to me likely ever 

 to get over it and return to the right method of observing, there- 

 fore, though with reluctance, as he was a diligent and useful as- 

 sistant to me in other respects, I parted with him." 



Thus Mr. David Kinnebrook fell a victim to the earliest dis- 

 covery of the difference of power of observation. 



How these differences were measured it would take too long to 

 relate. The results only can be stated, and for details reference 

 made to an article by Prof. Cattell in a recent number of " The 

 Popular Science Monthly" on "The Time it takes to Think," 

 and to one by Prof. Sandford, in the " American Journal of Psy- 

 chology," on the " Personal Equation." * 



Any act which depends upon sensation, such as returning a 

 tennis-ball or replying to a question, takes time. This act can be 

 separated into certain parts. There is the perception of the sensa- 

 tion, the decision to respond to it, and the act of motion. You 



* Vol. ii, No. l. 



