The Ferrier Lecture, 1962 Visual adaptation

Abstract

You will need no words of mine to convince you how precious are your eyes, for besides your experience of the incomparable richness that vision brings to human life, you will reflect that animal life, though clothed in bodies of fantastic diversity can seldom afford to be without sight of a sort, and generally possesses some good visual discrimination. The function of the eye and its nerves is roughly this: to collect light from various places in order to obtain information about rather distant surroundings, to discriminate the patterns of form and movement, and to initiate appropriate reactions. The vertebrate eye, like a photographic camera, collects light with a lens so that a replica of the outside world is formed as an image upon the retina. Here the pattern of light energy is transformed into a pattern of chemical change by the photosensitive pigments of the retinal film. But in the eye (unlike the camera) it is not sufficient for the picture to be ‘taken’ ; it may not lie dormant until it is convenient to develop and appreciate it, for the transmission of information to the brain is often a matter of the greatest urgency. Thus the visual pigment lies not spread in a structureless film but enclosed in a mosaic of active cells, the rods and cones, which constitute the ‘grain’ of the retinal image. Each of these cells can respond to light and give rise to a message that passes towards the brain. By the time the message is taken up by the fibres of the optic nerve we know that it is in the familiar pattern of impulses similar to those in all other long nerves. But we do not know the nature of the complex message transmitted from the rods to the optic nerves.