IV
All will have noticed that when we look intently at anything, all other objects become the more indistinct the further they are removed from the point of attention. This is because we have only one point in our eye in which vision reaches a maximum of acuteness. This point is called the fovea centralis, because it looks like a little dimple or funnel of two-tenths of a millimetre in diameter. If the image of an object falls at a distance of only a few millimetres from the fovea centralis, the eye can no longer accurately distinguish the colours. Red and green give an impression of palish yellow, violet appears blue. A little further distant, yellow and blue disappear completely, and only light and dark are perceived. This anatomical disposition of the elements destined to perceive the image and colour of objects obliges us to move the eye and bring it into relation with all parts of an object if we wish to examine it minutely. On this account no organ has such precise movements as the eye. If we look at our eye in a mirror, and move our head up and down, to the right and to the left, we see to our astonishment that the eye can remain fixed and motionless. Let the reader repeat this experiment in order to conceive the facility and precision with which the eye fixes on one point which we wish to look at attentively. The restlessness of the eye contemplating an unknown figure, the agitation which is visible in a man when he is afraid of another, and therefore examines him from head to foot in order to be ready to defend himself, or escape an impending danger, is an inevitable consequence of the structure of the eye, which cannot contemplate and embrace a wide field without moving.
When the object is not small enough to be embraced by the simple movements of the eye in its orbit, we bend and turn the head, or move the trunk to right or left; if that does not suffice, we move the whole body. Actors represent fear by exaggerating the attitude peculiar to one intently observing an object.
These movements are so spontaneous and natural, that it costs an effort to keep head and body still when looking at an object situated on one side of us. A feeling of profound contempt, hatred, or pride is necessary before we can pass close to a man with head stiff and erect.