HOW CUTTING INCREASES BRILLIANCY. Primarily the object of cutting a diamond is to make it more brilliant. So true is this that the usual form to which diamonds are cut has come to be called the _brilliant_. The adjective has become a noun. The increased brilliancy is due mainly to two effects: First, greatly increased reflection of light, and second, dispersion of light. The reflection is partly external but principally internal. Taking up first the internal reflection which is responsible for most of the white brilliancy of the cut stone we must note that it is a fact that light that is passing through any transparent material will, upon arriving at any polished surface, either penetrate and emerge or else it will be reflected within the material, depending upon the angle at which the light strikes the surface. For each material there is a definite angle outside of which light that is passing as above described, is _totally reflected_ within the material. [Illustration: FIG. 9. _AB_ represents the back surface of a piece of diamond. _CD_ is a line perpendicular to _AB_. Angle _CDE_ is about 24 degrees. Dotted line, _FDH_ represents the course taken by a ray of light which is totally reflected at _D_ in such fashion that angle _FDA_ equals angle _HDB_. Any light proceeding towards _AB_ but between _E_ and _C_, would fail to be totally reflected. Most of it would penetrate _AB_.] TOTAL REFLECTION. For diamond this _critical angle_, as it is called, is very nearly 24° from a perpendicular to the surface. If now, we shape a diamond so that most of the light that enters it from the front falls upon the first back surface that it meets, at an angle greater than 24° to a perpendicular to that surface, the light will be totally reflected within the stone. The angle at which it is reflected will be the same as that at which it meets the surface. In other words the angles of incidence and of reflection are equal. See Fig. 9 for an illustration of this point. THEORY OF THE "BRILLIANT." In the usual "brilliant" much of the light that enters through the front surface is thus totally reflected from the first rear facet that it meets and then proceeds across the stone to be again totally reflected from the opposite side of the brilliant. This time the light proceeds toward the top of the stone. See Fig. 10--(From G. F. Herbert-Smith's _Gem-Stones_).