Apparent Section of Oblate Spher­oid­al Mirror.

If the opaque screen be drawn a short distance from the mirror, the appearance of the section curve will seem to change, the bottom of the groove (Fig. 12) between the centre and edge advancing inwards, and the mound in the middle growing smaller. If the screen be pushed toward the mirror the reverse takes place, the central mound becoming larger, but the edge decreasing. The reason for these variations becomes apparent by considering the three diagrams, Fig. [14]. The dotted curve in each instance represents the real curve of the mirror described in the last paragraph, while the solid lines are circles drawn with radii progressionally shorter in a, b and c, and represent sections of three spherical mirrors whose focal lengths also progressively shorten.

Fig. 13.

Section of Spherical and Spheroidal Mirrors.

Fig. 14.

Relation of Spheres to Oblate Spheroid.

Fig. 15.