bring about the very best results. Dr. Alfred Mayer's method of getting
over the great difficulty of knowing when all four points are in contact
is quite simple. The standard plate is set on the box, _g_, Fig. 4, which
acts as a resonater. The screw, _a_, is brought down until it touches the
plate. When the pressure of the screw is enough to lift off either or all
of the legs, and the plate is gently tapped with the finger, a _rattle_ is
heard, which is the tell-tale of imperfect contact of all the points. The
screw is now reversed gently and slowly until the _moment_ the rattle
ceases, and then the reading is taken. Here the sense of hearing is
brought into play. This is also the case when the electric contact is
used. This is so arranged that the instant of touching of the point of
screw, _a_, completes the electric circuit, in which an electromagnet of
short thick wire is placed. At the moment of contact, or perhaps a little
before contact, the bell rings, and the turning of the screw must be
instantly stopped. Here are several elements that must be remembered.
First, it takes time to set the bell ringing, time for the sound to pass
to the ear, time for the sensation to be carried to the brain, time for
the brain to send word to the hand to cease turning the screw, and, if you
please, it takes time for the hand to stop. You may say, of what use are
such refinements? I may reply, what use is there in trying to do anything
the very best it can be done? If our investigation of nature's profound
mysteries can be partially solved with good instrumental means, what is
the result if we have better ones placed in our hands, and what, we ask,
if the _best_ are given to the physicist? We have only to compare the
telescope of Galileo, the prism of Newton, the pile of Volta, and what was
done with them, to the marvelous work of the telescope, spectroscope, and
dynamo of to-day. But I must proceed. It will be recognized that in
working with the spherometer, only the points in actual contact can be
measured at one time, for you may see by Fig. 6 that the four points, _a a
a a_, may all be normal to a true plane, and yet errors of depression, as