# File lib/captcha.rb, line 297
    def clean
      Dir.foreach( @image_dir ) do |entry|
        next if entry !~ /\.png$/
        if Time.now - File.stat( File.join( @image_dir, entry ) ).mtime > @clean_up_interval
          File.delete( File.join( @image_dir, entry ) )
        end
      end
    end