An on-line focusing procedure for a computer-controlled microscope employed in the measurement of human chromosome images is discussed. The method is based on an algorithm proposed by Mendelsohn and Mayall. A focusing accuracy of standard deviation less than 1/20th micron in the slide-objective distance has been achieved. Changes of the focus position with time, and for individual chromosomes within a metaphase, have been investigated. The method has proved insensitive to imperfections in the scanner photocathode. The method has also been used to determine the autofocusing accuracy of an analog metaphase-finding apparatus.
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Mason et al. (1975) studied this question.