// describes inside of a shape (similar to texture, but for inside, not surface)
  interior {
    ior 1.31              // index of refraction
    dispersion            // needs a color_map in the light source
    dispersion_samples 7  // accuracy of calculation (2..100) [7]
    caustics 1.0          // faked caustics (0..1)
    fade_power 1001       // values larger than 1000 give realistic exponential attenuation
    fade_distance 3       // distance where light reaches half intensity
    fade_color <0, .3, 0> // color for fading
    media {..media items..}
  }
