SUBROUTINE AIR ( FSET, DISP, SPACING, fitdisp, dispf )
C
C   Calculate the appropriate dispersions for the two color method.
C
      IMPLICIT UNDEFINED (A-Z)
      save
      INTEGER *4 I, J, K, FSET
      REAL *8 WAVE(2), WREF(2), T, P, E, INDEX(2), S1(2), S2(2)
      REAL *8 LAMBDA(4), FR1, FR2
      REAL *4 SPACING(9,2), DISP(4), dispf(4)
      logical fitdisp,forcedisp,first
      data first,forcedisp / .true., .false. /
      DATA WAVE / .550, .500 /
      DATA WREF / .700, .800 /
      DATA T, P, E / 273., 1013., 0. /
      write(6,'(a,5f8.3)') ' Wavelengths are: ', wave, wref(fset)
      if(first)then
       first=.false.
       fitdisp=.false.
       write(6,*)'Enter dispersion values!'
       write(6,*)'(0 0 0 0 for default; -1 -1 -1 -1 to fit)'
c      read(5,*)dispf(1),dispf(2),dispf(3),dispf(4)
       if(dispf(1).eq.-1)then
        fitdisp=.true.
        write(6,*)'OK, let us fit the dispersion constants.'
       else if(dispf(1).ne.0)then
        forcedisp=.true.
        write(6,*)'OK, let us use the new dispersion constants.'
       endif
      endif
      CALL AINDEX ( P, T, E, WREF(FSET), INDEX(2) )
      DO I = 1, 2
          CALL AINDEX ( P, T, E, WAVE(I), INDEX(1) )
          DISP(I) =  INDEX(1) / ( INDEX(1) - INDEX(2) )
          if(forcedisp)disp(i)=dispf(i)
          SPACING(1,I) = 0
          DO J = 1, 2
              FR1 = FLOAT(J) * WREF(FSET)
              DO WHILE ( FR1 .GT. 0. )
                  FR1 = FR1 - WAVE(I)
              END DO
              SPACING(4*J-2,I) =  FR1
              SPACING(4*J-1,I) =  FR1 + WAVE(I)
              SPACING(4*J  ,I) = -FR1
              SPACING(4*J+1,I) = -FR1 - WAVE(I)
          END DO
          DO J = 1, 9
              SPACING(J,I) = SPACING(J,I) * DISP(I)
          END DO
      END DO
      if(fset.eq.2)then
       call aindex ( p, t, e, wref(1), index(1) )
       disp(4) = index(1) / ( index(1) - index(2) )
       if(forcedisp)disp(4)=dispf(4)
      endif
      CALL AINDEX ( P, T, E, WAVE(1), INDEX(2) )
      CALL AINDEX ( P, T, E, WAVE(2), INDEX(1) )
      DISP(3) =  INDEX(1) / ( INDEX(1) - INDEX(2) )
      if(forcedisp)disp(3)=dispf(3)
      RETURN
      END