accuracy the time it takes to perceive, that is, the time passing from the moment at which an impression has reached consciousness until the moment at which we know what it is. In my own case about one twentieth of a second is needed to see a white light, one tenth of a second to see a color or picture, one eighth of a second to see a letter, and one seventh of a second to see a word. It takes longer to see a rare word than to see a common one, or a word in a foreign language than one in our native tongue. It even takes longer to see some letters than others.
The time taken up in choosing a motion, the "will-time," can be measured as well as the time taken up in perceiving. If I do not know which of two colored lights is to be presented, and must lift my right hand if it be red and my left hand if it be blue, I need about one thirteenth of a second to initiate the correct motion. I have also been able to register the sound-waves made in the air by speaking, and thus have determined that in order to call up the name belonging to a printed word I need about one ninth of a second, to a letter one sixth of a second, to a picture one quarter of a second, and to a color one third of a second. A letter can be seen more quickly than a word, but we are so used to reading aloud that the process has become quite automatic, and a word can be read with greater ease and in less time than a letter can be named. The same experiments made on other persons give times differing but little from my own. Mental processes, however, take place more slowly in children, in the aged, and in the uneducated.
It is possible, further, to measure the time taken up in remembering, in forming a judgment, and in the association of ideas. Though familiar with German, I need, on the average, one seventh of a second longer to name an object in that language than in English. I need about one quarter of a second to translate a word from German into English, and one twentieth of a second longer to translate in the reverse direction. This shows that foreign languages take up much time even after they have been learned, and may lead us once more to weigh the gain and loss of a polyglot mental life. It takes about two fifths of a second to call to mind the country in which a well-known town is situated, or the language in which a familiar author wrote. We can think of the name of next month in half the time we need to think of the name of last month. It takes on the average one third of a second to add numbers consisting of one digit, and half a second to multiply them. Such experiments give us considerable insight into the mind. Those used to reckoning can add two to three in less time than others; those familiar with literature can remember more quickly than others that Shakespeare wrote "Hamlet." In the cases which we have just been considering a ques-
- In all cases the time of association only is given, the time needed to see the one word and name the other having been subtracted.