One of the earliest instances of work of this kind was the introduction a few years ago of reaction-time tests in selecting girls for the work of inspecting for flaws the steel balls used in ball bearings. This work requires a concentrated type of attention, good visual acuity and quick and keen perception, accompanied by quick responsive action. The scientific investigator went into a bicycle ball factory and with a stop-watch measured the reaction-time of all the girls then at work. All those who showed a long time between stimulus and reaction-time were then eliminated. The final outcome was that thirty-five girls did the work formerly done by one hundred and twenty; the accuracy of the work was increased by sixty-six per cent; the wages of the girls were doubled; the working day was shortened from ten and one-half hours to eight and one-half hours; and the profit of the factory was substantially increased. [Sidenote: _Professor Muensterberg's Experiments_] To illustrate the methods employed and the importance of work of this kind, we quote the following from the recent ground-breaking book, "Psychology and Industrial Efficiency," by Professor Hugo Muensterberg, of Harvard University. This extract is an account of Professor Muensterberg's experimental method for determining in advance the mental fitness of persons applying for positions as telephone operators. Such information would be of immense value to telephone companies, as each candidate who satisfies formal entrance requirements receives several months' training in a telephone school and is paid a salary while she is being trained. [Sidenote: _Tests for Hiring Telephone Girls_] One company alone employs twenty-three thousand operators, and more than one-third of those employed and trained at the company's expense prove unfitted and leave within six months, with a heavy resulting financial loss to the company. The tests are numerous and somewhat complicated and require more time to conduct them than tests in other lines of work, but for these very reasons will be particularly illuminating. Professor Muensterberg says: