The paper I have presented to you here is a brief account of the work we have been doing in the past six years at Farnborough and B.L.E.U. In studying visual judgments during the past few years, we have been driven to one conclusion which is pretty well the same as that which Capt. Prowse put before you, namely, that we are reaching the limit of what the human being can do. Every visual task has a certain failure rate, and I think this rate depends on the value of V|ak, where V is the approach speed, a is the acceleration which the pilot is able and willing to apply during a corrective manoeuvre, and k is an index representing the goodness of the visual stimuli. The tendency is for V|a to increase as aircraft get larger and heavier, but we have hitherto managed to counteract this by improving the visual aids, i.e. by increasing k.