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its external surface, which is without glaze—under the magnifier it seems
rough, like fine sandpaper. The external surface stops the light as do
crushed transparent substances generally—as, for instance, common salt
—for the reason that the rays of light are so often reflected among the
particles on the surface that they are scattered into a multitude of crossed
reflections that appear white, and therefore can not enter. Thus it fol-
lows that whatever surface will not admit light will also refuse to allow
a reflected shade to pass ; consecjuently if the dentin can be restored with
a very high-fusing porcelain that becomes densest before it glazes, that
presents a vitreous surface resembling sandpaper, and of the color of
dentin, we have produced a foundation for the filling that will not absorb
sufficient light to cause a shadow, and which, if not glazed by a subse-
quent baking, will still retain the power to arrest transmitted light, no
matter how the colors and enamel are laid on afterward."
Another difficult problem is the formation of a shade. A shade for
the purpose of this work may be defined as a deficiency of light within an
illuminated region, caused by the interception of light by some body, or
a condition produced by a reflected light. When the light is incident on
the teeth or lips, it is partially excluded from all or a portion of inlays
in some of the teeth, and this causes the inlay to appear much darker.
This fact should be considered when selecting colors for an inlay. The
color of the tooth should be studied with the incidence of light at difl:'erent
angles and with the lips partially covering the tooth. In selecting colors
for inlays in the distal surfaces of laterals and cuspids, the hues of the
color should be lighter than for inlays on their mesial surfaces. If a
porcelain inlay is placed in approximation with a gold filling, it will
appear darker because a portion of the reflected shade from the gold
will be absorbed by the porcelain, then returned to the surface of the
inlay and reflected to the eye, which gives to the porcelain a saddened
appearance.
A change in color is often noticeable when an
inla}- is cemented into place. The inlay may be a
Tnflucnce
Of Cement on good match when the incidence of light is at such an
Color. angle as will permit its transmission. But when the
angle of incidence is changed, some portion of the
inlay may appear a dififerent color, caused by the formation of a shade.
In this phenomenon the cement is the chief factor of the color problem.
If it were transparent, it would not prevent the passage of light through
the tooth. If light penetrates a short distance into a body and is then
reflected, the surface generally appears to have the color of the reflected
rays. But since light is not transmitted by an opaque substance, the rays

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