Similar illusions take place in certain pieces of polished wood, chalcedony, and mother-of-pearl, where the surface is perfectly smooth. This arises from there being at that place a knot or growth, or nodule, of different transparency from the surrounding mass, and the cause of it will be understood from Fig. 20. Let _m_ _o_ be the surface of a mahogany table, _m_ A _o_ B a section of the table, and _m_ _n_ _o_ a section of a knot more transparent than the rest of the mass. Owing to the transparency of the thin edge at _o_, opposite to the candle S, the side _o_ is illuminated, while the rest of the knot is comparatively dark, so that, on the principles already explained, the spot _m n o_ appears to be a hollow in the table. From this cause arises the appearance of dimples in certain plates of chalcedony, called hammered chalcedony, owing to its having the look of being dimpled with a hammer. The surface on which these cavities are seen is a section of small spherical aggregations of siliceous matter, which exhibit the same phenomena as the cavities in wood. Mother-of-pearl presents the very same phenomena, and it is indeed so common in this substance, that it is nearly impossible to find a mother-of-pearl button or counter which seems to have its surface flat, although they are perfectly so when examined by the touch. Owing to the different refraction of the incident light by the different growths of the shell cut in different directions by the artificial surface, like the annual growth of wood in a dressed plank, the surface has necessarily an unequal and undulating appearance. [Illustration: _Fig. 20._]