It means that that a thing looks normal can have a clearly seen thing which exists in the distance. As lower anime shows, the light which comes from the triangle which exists in the distance is in a cornea and a crystalline lens (lens), and reaches the retina.
At this time, a lens is refracted in light, without becoming thick. This is called stigmatism.

Next, if it is going to see a thing, without a lens and a cornea becoming thick when a triangle approaches close at hand, a triangular image will be made at a place more distant than the retina. That is, the blurred triangle will be made at the retina.

Then, what should we do in order to project the image of this triangle on the retina vividly? What is necessary is just to adjust so that a cornea may become thick and a lens may also become thick.

How far is a last-minute distance where the triangle approaching an eye looks vividly think in stigmatism? For a 40-year-old person, it is about 25 cm. It will fade, if a triangle approaches an eye to the front from 25 cm.

From a very distant place to 25 cm, a thing appears distinctly. In order to see distinctly, the thickness of a cornea and a lens is adjusted.