4. The power to distribute tension economically--i. e., the ability to sustain it long enough, and to release it at the right moment (after the manner of the curves described on page 93), and to control properly the unavoidable variations which will occur.[AI] [Footnote AI: The mental state just described is probably essentially the same as that of the spiritualistic "mediums" when they are occupied with table-rapping and table-moving. In both cases concentration is very intense,----in other words, the field of attention is limited. We saw that this state not only favors the tendency toward involuntary movement, but on account of the absorption of the individual's attention by a certain limited content, the person will be unaware of the voluntary movements as they occur. And we are not necessarily here dealing with neurasthenic, hysteric, or other diseased nervous conditions. In the case of table-rapping there are movements of the hands, in our case there are those of the head. Our head, balanced as it is upon the cervical vertebral column, is continually in a state of unstable equilibrium and therefore peculiarly susceptible to movement-impulses of every kind. But I could induce not only movements of the head, but also of the arms and legs, and this by having the subject assume a posture which enabled him to hold arms or legs in as unstable a position as possible. He might stretch out his legs horizontally before him, or he could raise them vertically upward as in the hand-stand in gymnastic work. An extract from a treatise by Count A. de Gasparin,[94] which appeared about the middle of the last century, may serve to show how close the correspondence between the two processes, that of getting the table to rap and that of causing Hans to respond, really is. The report of this writer, based upon the detailed record of his tests in table-moving and table-rapping, closely parallels in many minute details the observations which were made in the course of our experimentation with Hans. The case is all the more remarkable when we bear in mind that this writer did not seek the cause of the phenomena, as we did, in involuntary movements, but thrusting aside this explanation, he posited the cause in the agency of some mysterious fluid. It may not be amiss to say that this as well as most other references were consulted after the present experiments and introspections had been completed. Of the page references preceding the following citations, the first always refers to the page in the French original, and the other, enclosed in brackets, to the parallel passage in the present monograph.