The lower pair of mirrors _c_, _d_, again twice reflect the light at the polarizing angle, and, in the position shown, pass it on to the ocular diminished only by the four reflections. But if the second pair of mirrors be rotated together about a line parallel to _b c_ as an axis the transmitted light begins to fade out, and when they have been turned 90°, so that their planes are inclined 90° to _a_ and _b_ (= 33° to the plane of the paper), the light is substantially extinguished. Thus by merely turning the second pair of mirrors the solar image can be reduced in brilliancy to any extent whatever, without modifying its color in any way. The typical form given to the polarizing eyepiece is similar to Fig. 129. Here _t__2 is the box containing the polarizing mirrors, _a b_, fitted to the draw tube, but for obvious reasons eccentric with it, _t__1 is the rotating box containing the “analysing” mirrors _c_, _d_, and _a_ is the ocular turning with it. Sometimes the polarizing mirrors are actually a pair of Herschel prisms as in Fig. 126, facing each other, thus getting rid of much of the heat. Otherwise the whole set of mirrors is of black glass to avoid back reflections. In simpler constructions single mirrors are used as polarizer and analyser, and in fact there are many variations on the polarizing solar eyepiece involving about the same principles. [Illustration: FIG. 129.—Polarizing Solar Eyepiece.] In any solar eyepiece a set of small diaphragms with holes from perhaps 1/64 inch up are useful in cutting down the general glare from the surface outside of that under scrutiny. These may be dropped upon the regular diaphragm of the ocular or conveniently arranged in a revolving diaphragm like that used with the older photographic lenses. The measurement of celestial objects has developed a large group of important auxiliaries in the micrometers of very varied forms. The simplest needs little description, since it consists merely of a plane parallel disc of glass fitting in the focus of a positive ocular, and etched with a network of uniform squares, forming a reticulated micrometer by which the distance of one object from another can be estimated. It can be readily calibrated by measuring a known distance or noting the time required for an equatorial star to drift across the squares parallel to one set of lines. It gives merely a useful approximation, and accurate measures must be turned over to more precise instruments. [Illustration: FIG. 130.—Diagram of Ring Micrometer.]