We called this chapter "Visual Acuity" and not "Limit" or "Power of Resolution",
because if we wanted to make a detailed study, we should make a study of optometry,
which would include biology, psychology and medicine.
This is different from the study of eventual prosthesi, like lenses and microscope,
which which we consider in future paragraphs.

In Optometry the Visual Acuity is divided in:

Acuity of Visibility

The human eye can distinguish, without any help of lenses, from a distance of 25 cm:
1) a small particle until 100µ;

Acuity of Resolution

2) The human eye can distinguish 2 particles of 100µ as separate particle
only of the distance between them is not less than 5µ;
If the 2 particles are closer than 5µ,
the human eye is merging them in one particle only, as shown below;

Therefore: the Acuity of Resolution is 5µ.


The sensibility-threshold of the human eye,
meant as the minimum flow of photons,
able to comunicate enough energy in order to
activate the chemical substances, like rhodopsin,
which allow the generation of the electrical impulse to transfer,
through the optical nerve, to the cerebral cortex,
is about 4 photons absorbed in a medium time of 0,15 seconds.