Astigmatism, also called stigmatism, is an eye condition where the cornea is somewhat elongated instead of round.
Airy, the astronomer, corrected his own astigmatism by means of a cylindrical lens.
The correction of astigmatism is in many cases a matter of considerable difficulty, but the results to vision almost always reward the trouble.
The aberrations of the third order are: (1) aberration of the axis point; (2) aberration of points whose distance from the Aberra- axis is very small, less than of the third order - the tions of deviation from the sine condition and coma here fall together in one class; (3) astigmatism; (4) curvature of the field; (5) distortion.
These anastigmatic lenses, which are manufactured up to X 40, are chromatically and spherically corrected, and for a middle diaphragm the errors of lateral pencils, distortion, astigmatism and coma are eliminated.
If measures are made by placing the image of a star in the centre of the disk of a planet, the observer may have a tendency to do so systematically in error from some acquired habit or from natural astigmatism of the eye.
In astigmatism, owing to differences in the refractive power of the various meridians of the eye, great defect of sight, frequently accompanied by severe headache, occurs.
It must be so designed as to give as flat an image as is possible consistently with freedom from astigmatism of oblique pencils.