WHAT ASPECT OF THE PROBLEM WAS MOST DIFFICULT?
     With an unlimited constraint on the number of surfaces or glass types,
     the correction of the primary Seidel aberrations was relatively easy.
     No one ever said we had to build this!  The most difficult aspect of
     the design was minimizing the secondary chromatic aberration.  Using
     glass manufacturers' partial dispersion maps to substitute glasses off
     the nominal dispersion line worked best in accomplishing this.  SF59
     was very effective in reducing secondary color, but its extremely high
     specific gravity minimized its effectiveness.  SFL56 was not as
     effective initially, but with a weight barely half that of SF59, the
     reduction in weight provided the freedom to achieve better overall
     correction.