ng 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
at _e_, or elevation, as at _b_, exist between them, so that the
instrument must be used over every available part of the surface if it is
to be tested rigorously. As to how exact this method is I cannot say from
actual experience, as in my work I have had recourse to other methods that
I shall describe. I have already quoted you the words of Prof. Harkness.
Dr. Hastings, whose practical as well as theoretical knowledge is of the
most critical character, tells me that he considers it quite easy to
measure to 1/80000 of an inch with the ordinary form of instrument. Here
is a very fine spherometer that Dr. Hastings works with from time to time,
and which he calls his standard spherometer. It is delicately made, its
screw being 50 to the inch, or more exactly 0.01998 inch, or within
2/100000 of being 1/50 of an inch pitch. The principal screw has a point
which is itself an independent screw, that was put in to investigate the
errors of the main screw, but it was found that the error of this screw
was not as much as the 0.00001 of an inch. The head is divide