If, by these methods, a point in the optic axis has been freed from aberration, it does not follow that a point situated only a very small distance from the optic axis can also be represented without spherical aberration.
On these principles Airy, by an elaborate mathematical investigation, successfully explained the interference patterns obtained with plates of quartz perpendicular to the optic axis.
With a crossed polarizer and analyser the rings are interrupted by a dark hyperbolic brush that cuts the plane of the optic axes at right angles, if this plane be at 45° to the planes of polarization and analysation - the so-called diagonal position - and that becomes a rectangular cross with its arms parallel and perpendicular to the plane of the optic axes when this plane coincides with the plane of primitive or final polarization - the normal position.
Crossed, inclined and horizontal dispersion are characterized respectively by a distribution of colour that is symmetrical with respect to the centre alone, the plane of the optic axes, and the perpendicular plane.
Then with an uniaxal plate perpendicular to the optic axis, the black cross is replaced by two lines, on crossing which the rings are discontinuous, expansion or contraction occurring in the quadrants that contain the axis of the quarter-wave plate, according as the crystal is positive or negative.
This is a plate made of two equal wedges of quartz, that can be moved over one another so as to vary its thickness, and are cut so that the faces of the plate are parallel to the optic axis, which in the first wedge is perpendicular and in the second is parallel to the refracting edge.
In thickness with its faces perpendicular to the optic axis is nearly opaque to light falling normally upon it, and a plate of this thickness parallel to the axis permits of the passage of a single stream polarized in a plane perpendicular to the principal section.
(Reference should be made to the article Crystallography for illustrations, and for applications of these phenomena to the determination of crystal form.) With an uniaxal plate perpendicular to the optic axis, the curves of constant retardation are concentric circles and the lines of like polarization are the radii: thus with polarizer and analyser regulated for extinction, the pattern consists of a series of bright and dark circles interrupted by a black cross with its arms parallel to the planes of polarization and analysation.
In the case of a biaxal plate perpendicular to the bisector of the acute angle between the optic axes, the curves of constant retardation are approximately Cassini's ovals, and the lines of like polarization are equilateral hyperbolae passing through the points corresponding to the optic axes.
When the rings are coloured symmetrically with respect to two perpendicular lines the acute bisectrix and the plane of the optic axes are the same for all frequencies, and the colour for which the separation of the axes is the least is that on the concave side of the summit of the hyperbolic brushes.
With a biaxal plate perpendicular to the optic axis in the diagonal position, the hyperbolic brush becomes an hyperbolic line and the rings are expanded or contracted on its concave side, with a positive plate, according as the plane of the optic axes is parallel or perpendicular to the axis of the quarter-wave plate, the reverse being the case with a negative plate.
757) have shown by a few biaxal crystals, such as sugar and Rochelle salt, the rotation produced by a given thickness being in general different, and in some cases of opposite sign for the two optic axes.
Wollaston, by blocking off one of the streams with a screen, sufficient lateral separation being obtained by combining two equal crystalline prisms cut differently with respect to the optic axis - an arrangement that achromatizes more or less completely the pencil that is allowed to pass.
Theoretically the best construction for prisms of this class is the following: a rectangular block of Iceland spar, of length about four times the width and having its end and two of its side faces parallel to the optic axis, is cut in half by a plane parallel to the optic axis and making an angle of about 14° with the sides; the two halves are then reunited with a cement whose refractive index is between the ordinary and extraordinary indices of the spar and as nearly as possible equal to the latter.
This refractive index should be equal to the greatest index of the plate, and with a biaxal plate the mean axis of optical symmetry should be parallel to its faces and in the normal section of the prisms, while with an uniaxal plate the optic axis should be in a plane perpendicular to this normal section.
Soda light, first sifted by passage through a plate of potassium bichromate, traverses in succession a lens, a Nicol's prism, and a glass plate half covered with a half-wave plate of quartz, that is cut parallel to the optic axis and has its principal section inclined at a small angle to that of the prism.
The former consists of two semicircular plates of quartz, perpendicular to the optic axis and of opposite rotations, placed so as to have a common diameter and having such a thickness that each rotates the plane of polarization of mean yellow light through the same multiple of 90°.
This is made of two plates of quartz, cut normally to the optic axis and of opposite rotations, placed the one in front of the other: the thickness of the one plate is fixed, while that of the other can be varied, as it is formed of two equal prisms that can be moved over one another along their common face.
It is essential that the objective is always brought before the lower end of the tube in such a way that the optic axis of the objective coincides with the optic axis of the rest of the system.
In many cases the stand is also movable round the optic axis.
Thus light, pressure, or mechanical stimulation acting on the retina and optic nerve invariably produces luminous impressions.
This consists of two plates of an uniaxal crystal of equal thickness, cut at the same inclination of about 45° to the optic axis and superposed with their principal planes at right angles.
If in the interference experiment with Fresnel's mirrors or biprism the slit be illuminated with white light that has passed through a polarizer and a quartz plate cut perpendicularly to the optic axis, it is found on analysing the light that in addition to the ordinary central set of coloured fringes two lateral systems are seen, one on either side of it.
When the light is circularly polarized or circularly analysed, a single plate gives two mutually enwrapping spirals, and similar spirals in circularly polarized light are obtained with plates of an active biaxal crystal perpendicular to one of the optic axes.
With the .orthogonal arrangement for illuminating and observing the beam of light traverses an extremely fine slit through a well-corrected system, whose optic axis is perpendicular to the axis of the microscope; the system reduces the dimensions of the beam to about 2 to 4 in the focal plane of the objective.
Forwards it covers, and has driven asunder, the optic lobes; backwards it hides the much shortened medulla oblongata.
5533; the optic sign is therefore positive.
Another cartilage or ossification, the posterior sclerotic ring, occurs within the walls of the posterior portion of the cup, and surrounds, especially in the Pici and in the Passeres, the entrance of the optic nerve.
From near the entrance of the optic nerve, through the original choroidal fissure, arises the much-folded pecten, deeply pigmented and very vascular, far into the vitreous humour.
The quadrate muscle adjusts the motion, and prevents pressure upon the optic nerve; during the state of relaxation of both muscles the nictitans withdraws through its own elasticity.
Curiously enough, however, they differ from the cephalic Molluscan eye in the fact that, as in the vertebrate eye, the filaments of the optic nerve penetrate the retina, and are connected with the re surfaces of the nerve-end cells nearer the lens instead of with the opposite end.