METHODS In this series of experiments twenty-four cats were used. Animals were anaesthetized with intraperitoneal thiopental sodium (40 mg/kg) and light anaesthesia was maintained throughout the experiment by additional intraperitoneal injections. The eyes were immobilized by continuous intravenous injection of succinylcholine; the employment of this muscle relaxant made it necessary to use artificial respiration. Pupils of both eyes were dilated and accommodation was relaxed by means of 1% atropine. Contact lenses used with a suitably buffered solution prevented the corneal surfaces from drying and becoming cloudy. The lids were held apart by simple wire clips.

A multibeam ophthalmoscope designed by Talbot & Kuffler (1952) was used for stimulation and viewing the retina of the left eye. Background illumination was usually about 0-17 log. metre candles (mc), and the strongest available stimulus was 1-65 log. mc Many sizes and shapes of spots of light could be produced, and these were well focused on the retina. Stimulus durations were of the order of 1 sec.