    When light passes through a surface either into or out of a dense medium,
 the path of the ray of light is bent.  Such bending is called refracyion.
 Normally transparent or semi-transparent surfaces in POV-Ray do not refract
 light.  Adding 'refraction 1.0' to the finish statement turns on refraction.

    NOTE: It is recommended that you only use 'refraction 0' or 'refraction
 1'.  Values in between will darken the refracted light in ways that do not
 correspond to any physical property.  Many POV-Ray scenes were created with
 intermediate refraction values before this 'bug' was discovered so the 'fea-
 ture' has been maintained.  A more appropriate way to reduce the brightness
 of refracted light is to change the 'filter' value in the colors specified
 in the pigment statement.  Also, 'refraction' does not cause the object to
 be transparent.  Transparency is only occurs if there is a non-zero 'filter'
 value in the color.

    The amount of bending or refracting of light depends upon the density of
 the material.  Air, water, crystal, diamonds all have different density and
 thus refract differently.  The 'index of refraction' or 'ior' value is used
 by scientists to describe the relative density of substances.  The 'ior'
 keyword is used in POV-Ray to specify the value.  For example:
       texture {
          pigment { White filter 0.9 }
          finish {
             refraction 1
             ior 1.5
          }
       }

    The default ior value of 1.0 will give no refraction.  The index of re-
 fraction for air is 1.0, water is 1.33, glass is 1.5, and diamond is 2.4.
 The file IOR.INC predefines several useful values for ior.

    NOTE: If a texture has a filter component and no value for refraction
 and ior are supplied, the renderer will simply transmit the ray through the
 surface with no bending.  In layered textures, the refraction and ior key-
 words MUST be in the last texture, otherwise they will not take effect.
