whole number of revolutions of the screw, _a_, the edge of which also
serves the purpose of a pointer to read off the division on the top of the
milled head, _e_. Still further refinement may be had by placing a vernier
here. To measure a plane or curved surface with this instrument, a perfect
plane 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