There will be this difference, however, that the parts of the surface of a body which appears to us hollow, and especially in relief, being but feebly varied in the greater number of contiguous parts, there will be generally a gradual diminution of the effects observed in the case in which we have studied the modifications of two plane homogeneous surfaces, lighted by diffused lights of unequal intensity. The sphere presents a remarkable example of the manner in which light is distributed over a convex surface, relatively to the eye of an observer, who views it from a given position. 282. I shall not occupy myself with this gradation of white light, from parts illuminated to those which do not appear so. I regard only the principal modifications, and take for examples the cases where they are as evident as possible. These modifications can be reduced to the four following:— _First modification_, produced by the maximum of white light which the surface of a coloured body is capable of reflecting. 283. Other things being equal, the more highly the surface of a body is polished, the more it will reflect white and coloured light. If we observe the surface of a stick of sealing-wax, suitably placed, we shall perceive a white stripe parallel to the axis of the cylinder, produced by so large a quantity of colourless reflected light that the red light reflected from this stripe is not appreciable by the eye. Thus the white light reflected by a coloured body may be of sufficient intensity to render the colour of the body in some of its parts imperceptible. _Second modification_, produced by those parts of a coloured surface which send to the eye, in proportion to the coloured light, less white light than the other parts differently lighted, or differently placed in relation to the spectator. 284. When the eye sees certain parts of the surface of a polished or uniformly coloured object which reflects to it proportionally to the coloured light less of white light than the other parts, the first parts will appear in most cases of a more intense tone of colour than the second. We will cite the following:— _Example 1._—A cylinder of red sealing-wax presents, proceeding from the white stripe mentioned above, a red colour deeper in proportion as less white light reaches the eye. Thus, in a certain position where the white stripe appears to be in the middle of the cylinder, the part most lighted will appear coloured, reflecting a red inclining to scarlet, while that which is the least lighted reflects a red inclining to crimson. _Example 2._—If the eye is directed into a gold vase of sufficient depth, the gold does not appear yellow as on the exterior surface, but of a red-orange; because less white light, in proportion to coloured light, reaches the eye in the first case than in the second. It is for this reason that the concave parts of gold ornaments appear redder than the convex.