can be seen. We all know how difficult it is to keep the hand or arm quite steady. They tremble, and we can feel the vibration. This can be demonstrated.
I have here a strong spring placed horizontally, and while it is firmly secured at one end I can pull upon the other. To show its vibrations, we have attached to the spring, and at right angles to it, a thin rod, the upper end of which is in the electric beam, and you see its shadow on the wall. When I pull as strongly as I can, you see the upper end of the rod vibrating. The muscles of my arm are in a state of strong contraction, of physiological tetanus, but they cannot remain permanently contracted. At one instant they relax a little, and the elastic recoil of the spring stretches them; then they contract for a moment, and so on. Thus they are vibrating.
While I was arranging this experiment I considered how I might be able to show you the time occupied by voluntary movements, say those of the fingers in writing a letter. It can be done in this way: Here is a tuning-fork worked by an electro-magnet, and vibrating about 240 times per second. I have an
