After psycho-physics, psycho-physiology was introduced by Wundt. Wundt, being a physiologist, applied the methods of study proper to physiological functions to psychical study. He did not make the exact metrical instrument his aim; but he measured nervous reactions exactly in _time_. Fechner's primitive researches made it possible to produce instruments so exact that they can measure the sound made by a drop of water falling from the height of a meter, while Wundt's researches have resulted in chronometers which can measure the thousandth part of a second. But the spirit did not correspond to the exactness of research--the results showed by their oscillations that nothing was being measured--that the object to be measured escaped. It will suffice to mention that in measuring the nervous currents in rate of transmission of impulse along the nerves and also in the ganglion cells of the spinal marrow, Exner arrived at a rapidity of eight meters, and Bloch at a rapidity of 194 meters, in the same unit of time. In spite of this startling contrast between the precision of the means of research and the huge variations in the results, which were shown by mathematical law to be absurd, experimental psychology carried on extensive studies, under the illusion that it rested upon a mathematical basis. It is from this science that a branch has been detached with which to penetrate into the school, for the purpose of giving spiritual help to the scholar, and fresh vigor to pedagogy. Methods of research are no longer merely those antiquated psycho-physical and psycho-physiological methods formerly in favor; experimental psychology, henceforth emancipated from its origins, has developed independently. It now relies on purely psychological tests for its researches, and although it does not exclude the methods adopted in the laboratory, and the use of such accurate and trustworthy instruments as the esthesiometer and the ergograph, the school itself has become the chief field of experiment. For example: one of the most familiar tests of attention is to give a printed page to be read over, with directions to strike out every _a_ on the page; the time taken to complete this task is measured by chronometer. Counting aloud from one to a hundred, and at the same time carrying on arithmetical operations in writing, is a measure of the distribution of the attention, provided the time taken be calculated by the chronometer, and all errors be noted. To make several persons perform similar exercises at the same time enables us to study comparative individual activities. In schools, exercises in dictation which have been previously determined, may be given to a group of scholars, care being taken to note the time occupied in performing the exercise and to compare the errors. This is also an easy and practical means of obtaining collective results.