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CUTTING INSTBUMENTS. 25
and study of the angles of instrnments in the market, and espe-
cially of the favorite instruments in dentists' cases, the following
angles in degrees of the astronomical scale were tried : 20, 40, 60,
80, 100 degrees. This was not found to give a good mental con-
ception of the relation of the angles to the quarter circle, which
was the principal factor sought. The centigrade circle was tried
with so much better results that it was adopted. It was found
also that a fairly even progression of numbers gave better angles.
It has proven very easy of mental grasp as compared with the
other scales or divisions of the circle. To convert centigrades
into degrees, multiply 360 by the number of centigrades and
divide by 100, or simply cut off two figures (move the decimal
point two figures to the left), as in calculating interest. The fol-
lowing is a comparison of the centigrades angle used, with
degrees
6 centigrades = 21 6 degrees.
.
"
12 = 43.2
" =
18 64.8
" =
23 82.8
" "
28 =100.8
"
80 =288.0
"
95 =342.0
The two last are angles of the cutting edges of the gingival mar-
gin trimmers. See Figure 23.
THE BOLEY GAUGE.
In dentistry, the metric system is much better suited to the
measurements necessary than feet and inches. For all of this
work, except the measurement of instruments, the Boley gauge.
Figure 18, is much the best device. It should be in the hands
of every student and dentist. It is widely used by watchmakers
and scientific men, and it is cheap enough for any one to own.
With it, one can make measurements to one-tenth millimeter
without difficulty. As there are approximately twenty-five milli-
meters to the inch, this is one-two hundred and fiftieth of an
inch. In doing this the Vernier is used. This is the short scale
on the sliding piece by which the jaws of the instrument are
opened. Notice particularly that the ten divisions of this short
scale — the Vernier — are equal to nine divisions of the prin-
cipal scale on the instrument bar. When, in reading a measure-
ment, it is found not to coincide with a division of the instrument
bar, but is plus a part of a division, the division of the Vernier
to the right that is opposite a division of the bar, gives the tenths
CUTTING INSTBUMENTS. 25
and study of the angles of instrnments in the market, and espe-
cially of the favorite instruments in dentists' cases, the following
angles in degrees of the astronomical scale were tried : 20, 40, 60,
80, 100 degrees. This was not found to give a good mental con-
ception of the relation of the angles to the quarter circle, which
was the principal factor sought. The centigrade circle was tried
with so much better results that it was adopted. It was found
also that a fairly even progression of numbers gave better angles.
It has proven very easy of mental grasp as compared with the
other scales or divisions of the circle. To convert centigrades
into degrees, multiply 360 by the number of centigrades and
divide by 100, or simply cut off two figures (move the decimal
point two figures to the left), as in calculating interest. The fol-
lowing is a comparison of the centigrades angle used, with
degrees
6 centigrades = 21 6 degrees.
.
"
12 = 43.2
" =
18 64.8
" =
23 82.8
" "
28 =100.8
"
80 =288.0
"
95 =342.0
The two last are angles of the cutting edges of the gingival mar-
gin trimmers. See Figure 23.
THE BOLEY GAUGE.
In dentistry, the metric system is much better suited to the
measurements necessary than feet and inches. For all of this
work, except the measurement of instruments, the Boley gauge.
Figure 18, is much the best device. It should be in the hands
of every student and dentist. It is widely used by watchmakers
and scientific men, and it is cheap enough for any one to own.
With it, one can make measurements to one-tenth millimeter
without difficulty. As there are approximately twenty-five milli-
meters to the inch, this is one-two hundred and fiftieth of an
inch. In doing this the Vernier is used. This is the short scale
on the sliding piece by which the jaws of the instrument are
opened. Notice particularly that the ten divisions of this short
scale — the Vernier — are equal to nine divisions of the prin-
cipal scale on the instrument bar. When, in reading a measure-
ment, it is found not to coincide with a division of the instrument
bar, but is plus a part of a division, the division of the Vernier
to the right that is opposite a division of the bar, gives the tenths
