#optical_glass__opticalglas  clear homogeneous glass of known refractive index; used to make lenses
  supertype:  #glass__glas  a brittle transparent solid with irregular atomic structure
  subtype:  #optical_crown__opticalcrown__crown_glass__crownglas  optical glass of low dispersion and low refractive index
  subtype:  #optical_flint__opticalflint__flint_glass__flintglas  optical glass of high dispersion and high refractive index

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