Since this value is the same for optical and acoustical, for successive and simultaneous impressions, it surely denotes some psychical constant independent of any special sense. And in fact in using different kinds of impressions we always arrive at the same result. The number six with very minor variations denotes the maximum of simple impressions that can be grasped by attention. If we choose syllables of any form, that are not combined into words, and if we read out a row of such to an observer, and require him to repeat them, we find that a correct repetition is possible with a row such as the following:-- ap ku no li sa ro Whereas it is not possible with a row like this:-- ra po su am na il ok pu We notice that even with seven such senseless syllables the repetition is generally unsuccessful. We may by practice become successful with seven syllables. This is obviously exactly the same result as we obtained above with our rows of metronome beats. There still remains another phenomenon that coincides with this result. It is the more worthy of note since it belongs to a third sense, namely the sense of touch, and since it was discovered from practical considerations quite independent of psychology. There had been many futile attempts to discover the most useful method of printing for the blind, before Braille, a French teacher of the blind, about the middle of last century solved this important practical problem. He himself had become blind, and was therefore in a better position than others to make sure of the requirements that were necessary, by means of experiments upon himself. He came to this result, that, first of all, groups of distinct points were the only suitable means of establishing letter-signs that could be easily distinguished, and that, secondly, not more than six definite points were to be used for one letter. These points must not spread over an extent greater than that which can be covered by the sense of touch, if the symbols are to be distinguished by the fingers of the blind with ease and certainty. He decided for an arrangement of points as seen in Fig. I., out of which the alphabet for the blind was arranged:-- [Illustration: Fig. I - II - III.]