Generally, the excitability of the vaso-motor nerves is the same in both halves of the body, and we all experience during strong emotions a feeling of cold, due to the contraction of the vessels, and spreading over the whole body, as though a cold sheet were being wrapped round our limbs and pressed upon our heart; this giving us an impression which one might call a mingling of several indefinite and varying impressions, as of darkness, cold, and of a dull, deep noise. The impression is generally more perceptible in the head and back, more rarely in the legs. Sometimes these contractions of the vessels take place without our knowing the cause; the popular superstition says that death is loitering near. It is one of those contractions arising spontaneously, like the involuntary, sudden starts to which we are often subject in bed before falling asleep. II Until recently no one had thought of studying the circulation in hands or feet, since even the most practised eye cannot with certainty distinguish the minimum variations in the colour of the skin, and because the thermometer applied to the surface of the body cannot accurately measure it. It occurred to me that one might easily attain this object by measuring the volume of the hand. I took a long, narrow bottle, and broke out the bottom of it. Into it the hand and a good part of the fore-arm must be introduced, and the bottle closed hermetically with putty. In the neck of the bottle I fastened a stopper, through which a long slender glass tube passed, and filled bottle and glass tube with tepid water. I thought if a greater quantity of blood flows into the hand, an amount of water corresponding to the increased quantity of blood will be forced out of the bottle, and, on the contrary, if the blood-vessels contract and the hand becomes smaller, the water contained in the slender tube which passes through the stopper will flow into the bottle. The first experiment, which I made on my brother, convinced me at once that I had discovered the right method, although at that time I was very far from imagining that I should be able to raise my simple apparatus to the dignity of a scientific method, and with it to add a chapter to the treatises of physiology. I shall not detain the reader with a description of the perfecting of this instrument, to which I gave the name of plethysmograph, or meter of changes of volume. A few months after making the first experiments on my brother, I returned to Leipzig to see the celebrated physiologist Ludwig, in order to tell him that I had thought out a very simple instrument, by means of which interesting circumstances in the circulation of the blood in man might be noticed. I shall always remember with deep emotion the satisfied look with which he examined the sketches which I, with trembling hand, drew upon the paper in order to make myself intelligible, his sincere pleasure, and the words with which he encouraged me to complete my studies in his laboratory.