Page:Popular Science Monthly Volume 35.djvu/770

744 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 understood 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 record very accurately the moment at which the eclipse was complete. 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 rapidity with which each man observed and recorded his observation, and those differences can now be measured. This was not appreciated at first, for we find that the result of this discovery of a difference between the records of the two observers was very unfortunate 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 August 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, therefore, though with reluctance, as he was a diligent and useful assistant to me in other respects, I parted with him."

Thus Mr. David Kinnebrook fell a victim to the earliest discovery 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 Psychology," 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 sensation, the decision to respond to it, and the act of motion. You