; nMG is fused quartz index from the Melles Griot Catalog
; lam is in nm
nMG = $  ; lam is in nm
 SQRT(1+(0.6961663*(lam/1000.)^2)/((lam/1000.)^2-(0.0684043^2))   $
     +(0.4079426*(lam/1000.)^2)/((lam/1000.)^2-(0.1162414^2))+    $
      (0.8974794*(lam/1000.)^2)/((lam/1000.)^2-(9.896161^2)))