Formation of Circles of Diffusion
An illustration depicting the formation of circles of diffusion. “From point A luminous rays enter the eye in the form of a cone, the kind of which will depend on the pupil. Thus it may be circular, or oval, or ever triangular. If the pencil is focused in front of the retina, as at d, or behind it as it as at f, or, in other words, if the retina of being at F; be in the position G or H, there will be a luminous circle or a luminous triangular space, and many elements of the retina will be affected. The size of these diffusion circles depends on the distance from the retina of the point where the rays are focused: the greater the distance, the more extended will be the diffusion circle” (Britannica, 132).
Encyclopaedia Britannica (11th ed., vol. 28) (New York, NY: The Encyclopaedia Britannica Company, 1910)