function Im=subImages(fileIndex,theDir,fileprefix);
%function Im=sumImages(fileIndex,theDir);
%adds tif files in fileIndex, under theDir, if no directory assumes current
%doesn't use filenumbers, just the order in which they appear in the
%directory
%minus sign subtracts

if nargin == 1
    theDir = '.'
end
if nargin == 3
    ImFiles =dir(strcat(theDir,'/',fileprefix,'*.tif'));
else
    ImFiles =dir(strcat(theDir,'/*.tif'));
end
Im = zeros(195,487);

for j=1:26
    fprintf('adding file %s\n',ImFiles(j).name);
    Tot_counts_ON(j)=sum(sum(imageread(strcat(theDir,'/',ImFiles(j).name),'tif',[487,195],32)));
    Tot_counts_OFF(j)=sum(sum(imageread(strcat(theDir,'/',ImFiles(j+1).name),'tif',[487,195],32)));    
end