Fig. 127.—Solar Diagonal.
Any reflection from the lower polished surface is turned aside out of the field, while the remainder of the radiation passes through the prism C and is concentrated below it. To prevent scorching the observer the lower end of the tube is capped at E, but the cap has side perforations to provide circulation for the heated air. Using such a prism, the remnant of light reflected can be readily toned down by a neutral tinted glass over the ocular.
In the telescopes of 3 inches and less aperture, and ordinary focal ratio, a plane parallel disc of very dark glass over the ocular gives sufficient protection to the eye. This glass is preferably of neutral tint, and commonly is a scant 1/16 inch thick. Some observers prefer other tints than neutral. A green and a red glass superimposed give good results and so does a disc of the deepest shade of the so-called Noviweld glass, which is similar in effect.
With an aperture as large as 3 inches a pair of superimposed dark glasses is worth while, for the two will not break simultaneously from the heat and there will be time to get the eye away in safety. A broken sunshade is likely to cost the observer a permanent scotoma, blindness in a small area of the retina which will neither get better nor worse as time goes on.
Fig. 128.—Diagram of Polarizing Eyepiece.
Above 3 inches aperture the solar prism should be used or, if one cares to go to fully double the cost, there is nothing more comfortable to employ in solar observation than the polarizing eye piece, Fig. 128. This shows schematically the arrangement of the device. It depends on the fact that a ray of light falling on a surface of common glass at an angle of incidence of approximately 57° is polarized by the reflection so that while it is freely reflected if it falls again on a surface parallel to the first, it is absorbed if it falls at the same incidence on a surface at right angles to the first.
Thus in Fig. 128 the incident beam from the telescope falls on the black glass surface a at 57° incidence, is again reflected from the parallel mirror b, and then passed on, parallel to its original path, to the lower pair of mirrors c, d. The purpose of the second reflection is to polarize the residual light which through the convergence of the rays was incompletely polarized at the first.
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.