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22 THE TECHNICAL PKOCEDUBES IN FILLING TEETH.

RULES FOE FORMING CONTRA-ANGLES. RECAPITULATION.

1st. All blades, the angle and length of which will bring the
cutting edge more than three millimeters from the central line
of the shaft, should be contra-angled.
2d. All instruments with angles of 12 centigrades or less,
when requiring contra-angles, should be binangle contra-angles.
3d. All instruments with angles of more than 12 centi-
grades, when requiring contra-angles, should be triple-angle
contra-angles.
4th. When the contra-angle is used, the cutting edge of the
instrument should be brought within two millimeters of the cen-
tral line of the shaft, or better — when the contra-angle is used,
the working edge should be brought just so near the central line
of the shaft that, when the instrument is laid edge downward
upon a plane surface (as the top of a table), the edge should just
touch, but not actually rest upon the surface.
Formula Names.
Formula names have been adopted for the cutting instru-
ments which describe each individual instrument so accurately
that each one may be known when its class and formula is spoken
or written. This is necessary in order that a teacher or writer
may be understood when speaking of the use of particular instru-
ments, and that students and dentists may speak intelligently of
these matters to each other.
These formula names are formed upon the same principle
as that used by the carpenter in naming his chisels or augers, as
half-inch chisel, one-inch chisel, three-quarter-inch auger, etc. But
to sufficiently describe the point of an excavator so that the par-
ticular instrument will be known at sight, it is necessary to give
three measurements. In all of this, the metric sj^stem of meas-
urement is used and there are three distinct units: One for
width of blade, the tenth of a millimeter; one for length of
blade, the millimeter; one for the angle of blade, the centigrade.
We give first the class name, as hatchet, hoe, spoon, etc., and
then give the formula of the point or working part. This for-
mula consists of the measurement, first, of the width of the blade
in tenths of a millimeter used as the unit; second, of the length
of the blade in millimeters; third, the angle of the blade with its
shaft or handle, in centigrades, or hundredths of the circle. Note
particularly that the width and length make up the size of the
blade. Also, in order tlint the individual instruments of tlie set