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1. The mesopic range, defined for cats with fully dilated pupils, extends from -1·0 to +1·4 log cd/m². The technique employed involves measurement of the relative shift in threshold with adaptation level, between two wave-lengths respectively selective for cones and rods at threshold. The magnitude of this shift is compared with that predicted from the Dartnall nomogram for visual pigments 556 and 507 (the in situ cone and rod absorption maxima).

2. The result is based on monochromatic increment threshold determinations for 149 on-centre or off-centre fibres, isolated in the optic tract posterior to the chiasma, under a variety of adaptation levels. All units included receive dual input — from 500 nm rods and from a single class of cones with maximum sensitivity at 556 nm. No units receive input from 450 nm cones; six units with input mediated purely by rods are excluded from the sample.