;+
; NAME:
;	ThomsonMidpointFnc
; PURPOSE:
;	Internal use by ThomsonMidpoint.
; CALLING SEQUENCE:
	FUNCTION ThomsonMidpointFnc, S, arg				; Internal use only
; INPUTS:
;	S	scalar; type: float
;			distance along line of sight in solar radii
;	arg	array(4}; type: float
;			[R,E,udark,halfint]
;			R = heliocentric distance observer in solar radii
;			E = elongation in radians
;			udark = limb darkening constant
;			halfint = half the intensity from 0 to infinity
; OUTPUTS:
;	R	scalar; type: float
;			integrated intensity upto distance S - halfint
; INCLUDE:
	@compile_opt.pro		; On error, return to caller
; CALLS:
;	ThomsonLOSRomb
; PROCEDURE:
;	Returns the difference between line-of-sight integrated intensity
;	up to specified distance S and half the integrated intensity over the
;	entire line of sight. I.e. the distance S which makes this function zero is
;	what ThomsonMidpoint is looking for.
;	No density function is passed to ThomsonLOSRomb, so a 1/r^2 density is assumed.
; MODIFICATION HISTORY:
;	JAN-1997, Paul Hick (UCSD)
;-

RETURN, ThomsonLOSRomb(arg[0],arg[1],udark=arg[2],upper=S)-arg[3]
END