Aberration

ABERRATION (from Latin aberrare - to go astray, to deviate), in biology - any minor deviation from the normal structure in an organism; the concept of A. is difficult to distinguish from the concept of 'variations,' and often the word A. was used as a synonym for a small variation; nowadays the concept of A. includes the meaning of a deviation having a more or less 'pathological character,' but in no way harming the viability of the organism; in this sense, A. is a very weak degree of anomaly. Aberration in physics - a term used in several meanings. Spherical A. - a phenomenon observed when light rays pass through glasses bounded by spherical surfaces, as well as when light is reflected by mirrors of the same shape (parabolic mirrors do not give aberrations). When rays are focused by glass or a mirror, the position of the focus depends on which part of the glass refracted (or reflected) the corresponding rays: central rays focus further away, while peripheral rays focus closer to the glass. The distance between the focus of the middle rays and the focus of the extreme rays is called the longitudinal A. of the glass. The transverse A. is the radius of the circle which the extreme rays trace on a plane drawn through the focus of the middle rays, perpendicular to the optical axis. Spherical A. can be reduced by cutting off with a diaphragm a narrow beam of only central rays. Spherical A. is completely eliminated in glasses called aplanats (see). -- Chromatic A. is observed when rays pass through optical glasses; mirrors do not give it. This phenomenon consists in that the colored rays that make up white light are gathered (independently of spherical A.) not in one point - each spectral color has its own focus. Due to chromatic A., the dark and light parts of images of objects are separated from each other by rainbow borders. The degree of divergence of paths of colored rays (so-called dispersion) depends on the type of glass, but by no means is proportional to its refractive power. This is the basis for the design of achromatic glasses (see Achromatism). Aberration of the eye occurs in the form of chromatic and spherical A. The chromatic A. of the eye, known to Newton, is caused by the different refrangibility of light rays of different wavelengths. Due to this, rays of different colors emitted by a point source of light, after refraction in the eye, do not gather in one common focus, but give a successive series of separate foci located in a space of about 0.5 mm. Red rays, which are refracted most strongly, are focused at a point closest to the front plane of the eye, immediately behind it lies the focus of yellow rays, then green, blue, and finally, furthest away - the focus of the least refracted violet rays. Helmholtz found for the schematic eye the distance between the foci of red and violet rays to be 0.434 mm. The insignificance of chromatic A. of the eye compared to artificial optical instruments is explained by the fact that the dispersion of the refractive media of the eye is much less than the dispersion of glass. Under normal physiological conditions, chromatic A. of the eye is not noticeable, and special methods are required to detect its presence. The diameter of the circles of diffusion in the eye (according to Helmholtz) turned out to be 0.0426 mm and, thus, quite capable of causing blurring of vision. The reason for the unnoticeability of A. under physiologically normal conditions, as Helmholtz showed, is the peculiar distribution of brightness in the circles of diffusion - namely, the brightness in the center can be considered infinitely large compared to the brightness of the periphery. - The cause of spherical A. of light rays in the eye is that monochromatic rays, after passing through a spherical refracting surface, do not quite gather in one point. The phenomena related to this include, for example, the unequal distinctness of vision of horizontal and vertical lines from a series of lines intersecting at one point, the formation by point sources of light of star-shaped figures, etc. The causes of this are: 1) unequal curvature of the refracting surfaces of the eye in different meridians, as well as in the plane of the same meridian (see Astigmatism), 2) not quite perfect transparency of the refractive media of the eye, due to which light rays experience reflection from the morphological elements of the refractive media, 3) the smallest irregularities at the edge of the pupil, and finally 4) the diffraction of light, caused by the presence of the pupil as a diaphragm.