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386 RESTORATION OF TEETH BY CEMENTED INLAYS.

ill four of a colorless high-fiisino; powder to that part of the mixed
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eiiaiucl which is to be used for the first baking. The unfused particles
extend across the matrix in every direction, making what is practically
an internal investment. The slight lightening thus occasioned is
entirely overcome by the second coat, and the jjropcr contour is obtained
in fewer bakings. When handling small fillings, the pliers and cavity
may be advantageously kept wet up to the time of insertion, as capillary
attraction will prevent the filling being dropped and lost. To place a
tiny filling on the operating-case in the same relative position that it
will take in the tooth prevents mistakes as to which side should go in
first.
In addition to the classification—labial, buccal, approximal, contour,
etc. — porcelain fillings are to be considered in regard to their position
in the mouth, viz. fillings that keep their color when cemented into
place, and those that will be darkened by the consequent shadow. Unless
these classifications are understood, many a well-matched porcelain inlay
will end by appearing dark and unsightly in the mouth. Color varia-
tions are met similar to those that are seen upon examining a piece
of window-glass. The surface may be nearly colorless while the edge is
dark green. The color of porcelain fillings is dependent u})on the per-
fection with which the light is reflected to the eye of the observer. For
instance, in a perfect light, yellow porcelain is yellow, because all of the
other rays that make up light are absorbed and only the yellow are
reflected to the eye. If the light be gradually decreased, fewer yellow
rays will be reflected, and the color will become darker ; when there is
no light reflected the porcelain will appear black. The more perfect the
front and side lights in porcelain fillings the less will be the shadow-
variations in color. Take, for example, a simple labial cavity, as illus-
trated in Fig. 367. If this extend into the dentin sufficiently deep to
prevent the color of the oxyphosphate of zinc or of any other cement
shining through it, the correct color of the porcelain will be given.
If, however, we place this well-matched material on the approximal
surface of the tooth, as in Fig. 368, with an adjacent tooth shutting
off" direct reflection, and thus allowing only indirect rays to meet the
eye of the observer, the color will be lost in shadow, and from having
been a perfect match, or nearly so, the shade w'ill assume a dull lead
color. Also, if in Fig. 367 the labial cavity should penetrate entirely
through the tooth, through the palatal enamel, the inlay would present
a problem of almost insurmountable difficulty ; for nearly all of the
direct rays would pass through it and would be lost in the shadows
of the mouth, while the side lights would be shut off by the non-
transparent but necessary zinc cement. This difficulty may be over-
come by placing two fillings, one on the palatal and one on the labial