method. The horizontal distances, since they are determined hy the rate at which the paper moves, correspond to the time. For example, if the paper moves an inch per minute, then, if we measure up a horizontal line the number of inches along which the tracing continues to rise, we can fix the points corresponding to a given time, and measure the height to which the pen had risen at any given instant, and also the rate at which it rose.
The vertical motion of the pen depends on the rise and fall of the test-tube M, which in its turn is caused by the out- and in-flow of the water surrounding the arm. Hence, a rise in the curve corresponds to an expansion, a fall to a contraction of the forearm. There are other details and precautions in the construction and use of the apparatus requisite to secure entire accuracy, for which I must refer to the original memoir. Modifications of the apparatus have been made by Ludwig and Kronecker, and a very essential improvement has been suggested and used by Dr. H. P. Bowditch, of Boston. These improvements are important to the experimenter, but have only a subsidiary interest for the general reader. The plethysmograph can be employed for many purposes. For example, it may serve as a very perfect thermometer, by applying it to record the changes of a closed volume of air, which of course increases or diminishes according as the temperature rises or falls.
With the aid of the plethysmograph Mosso discovered that the activity of the brain is directly connected with the circulation of the forearm. The importance of this fact will be better understood, if it is considered that we here have a physical phenomenon we can accurately measure, directly connected with mental phenomena we can not measure.
It is well known that the functional activity of an organ of the human body is accompanied by increased activity of the circulation, and this is also true of the brain. Since the total amount of blood is not subject to rapid and sudden changes, it is evident that, when an active part receives more blood, other parts must receive less. When the brain is at work, blood is withdrawn from the arm, which therefore becomes smaller.
In order that the volume of the forearm should remain constant, absolute tranquillity is necessary; the slightest movement of the mind suffices to disturb the equilibrium of the vascular system. The plethysmograph is an instrument with which we can record even emotions not depicted by any expression of the countenance, or revealed only by unnoticeable changes in the beating of the heart or of the respiratory movements. "Therefore," says Mosso, "I think it my duty to commence with a tracing that represents the sentiment of veneration which I felt in the presence of my beloved master. Behold, in fact, the contraction of the vessels produced by the entrance of Professor Ludwig, every time he honored the researches made upon myself with
