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Out-of-focus images tend to move and split when obstructions are passed through or near the nodal planes of the image forming lens, while in-focus images do not show such effects. Geometrical optics is used to produce a qualitative explanation of these effects with the object of utilizing them for automatic focusing and rangefinding devices. Several problems in such devices are briefly discussed and a proposed automatic rangefinder is described.

The Fe(59) method of Suit (1957) has been used to estimate total bone marrow cellularity in 10 haematologically normal patients. A mean figure of 10.4 x 10(9) nucleated cells/kg. body weight was obtained. Simultaneous estimates from marrow obtained by rib excision produced a mean value of 11.1 x 10(9) nucleated cells/kg. Previous work by this and other methods is reviewed. While some results agree well with those of the present work, one is higher by 50% and another by 300%. Corrections are suggested which reduce these figures and bring all published results within a small range. Reasonably accurate determinations of total marrow cellularity in disease will not be possible until more is known of marrow Fe(59) uptake in abnormal states.