function peak_lambda = planck_peak(start_lambda, end_lambda, temp, epsilon)
%
% Given an interval in wavelength and a temperature, find the peak wavelength
% of the Planck function within the interval.  Iterate until the fractional
% change in peak wavelength is less than epsilon.
%
% Arguments:    
%             start_lambda     (input)     starting point of interval (m)
%
%             end_lambda       (input)     ending point of interval (m)
%
%             temp             (input)     temperature of Planck function (K)
%
%             epsilon          (input)     fractional change in guessed 
%                                              peak wavelength at which we
%                                              stop looking
%
%             peak_lambda     (output)     peak wavelength of Planck function
%                                              within the interval (m)
%
% We'll use a bisection approach to find the peak.  At each iteration,
% we use the value of the slope at the current wavelength in order to
% decide if the peak lies in the lower or upper half of the current 
% interval.  In order to find the slope, we evaluate the Planck function
% at the current wavelength, and at another neighboring wavelength.
%
% MWR 3/31/2002
%

% check input values for validity

% this is the name of the Planck function we'll call

% in order to find the slope near the current wavelength, we'll check
%   the function's value at the current wavelength, AND at a wavelength
%   which is this fraction of the current interval away from it.

% prepare to enter main loop in which we search for the peak

% enter the main loop

  % use the middle of the current interval as the current guess

  % pick a neighboring wavelength

  % evaluate the Planck function at the current wavelength, and at
  % the neighboring wavelength
  
  % decide which half of the current interval contains the peak
  %   and move the appropriate boundary
  %  (and check to see if we're at the peak right now)
  
  
  % check to see if the fractional change since last time is small
  %   enough that we can quit
  
% end of main loop
  
% set the output argument "planck_peak" to the current wavelength