The figure below gives an opportunity to get acquainted with another imperfection of our eyes astigmatism. If you look at him with one eye, then from the four letters of this inscription is not all that likely to seem equally black. Notice what letter only blacker and turn the picture sideways. Unexpected change: the black letter will become gray and black area seems now another letter.
Look at this inscription one eye. One of the letters you will have more black than the rest.
In fact, all four letters are equally black, they have only hatched in different directions. If the eye was as perfectly arranged as expensive glass lenses, the direction of the strokes is not reflected in the blackness of the letters. But our eyes in different directions is not quite the same refracts rays, but because we can't immediately see equally clearly vertical, horizontal, and oblique lines.
Rarely anyone's eyes are completely free from this drawback, and some people astigmatism reaches such a strong degree that significantly interferes with vision, thus reducing its severity. Such persons need to see clearly, to use special glasses.
Around the eyes there are also other organic defects, which in the manufacture of optical devices wizard can be avoided. The famous Helmholtz expressed about these shortcomings as follows: “If any optician wanted to sell me a tool that has such drawbacks, I would consider himself entitled in the most dramatic way to speak about the neglect of his work and return to him the instrument of protest”.
But in addition to these illusions, which are due to a known fault, our eyes respond as a series of deceptions that have quite different causes.