ne or perfect spherical surface of known radius must be used to
determine the zero point of the division. Taking for granted that we have
this standard plate, the spherometer is placed upon it, and the readings
of the divided head and indicator, _d_, noted when the point of the screw,
_a_, just touches the surface, _f_. Herein, however, lies the great
difficulty in using this instrument, _i.e._, to know the exact instant of
contact of the point of screw, _a_, on the surface, _f_. Many devices have
been added to the spherometer to make it as sensitive as possible, such as
the contact level, the electric contact, and the compound lever contact.
The latter is probably the best, and is made essentially as in Fig. 5.

[Illustration: FIG. 11.]

I am indebted for this plan to Dr. Alfred Mayer. As in the previous
figure, _a_ is the screw; this screw is bored out, and a central steel pin
turned to fit resting on a shoulder at _c_. The end of _d_ projects below
the screw, _a_, and the end, _e_, projects above the milled head, and the
knife edge or pivot point rests against the lever, _f_, which in turn
rests against the long lever, _g_, the point, _h_, of which moves along
the division at _j_. It is evident that if the point of the pin just
touches the plate, no movement of the index lever, _g_, will be seen; but
if any pressure be applied, the lever will move through a multiplied arc,
owing to the short fulcri of the two levers. Notwithstanding all these
precautions, we must also take into account the flexure of the material,
the elasticity of the points of contact, and other idiosyncrasies, and you
can readily see that practice alone in an instrument so delicate will
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 touchi