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FIG. 17.—SUN ON MARCH 5, 1873. (FROM A DRAWING BY S.P. LANGLEY.)

My attention was first particularly directed to the subject in 1870 (shortly after the regular study of the Photosphere was begun at the Allegheny Observatory by means of its equatorial telescope of thirteen inches’ aperture), with the view of finding out what this vaguely seen structure really is. Nearly three years of constant watching were given to obtain the results which follow. The method I have used for it is indicated in the drawing (Fig.13), which shows the preliminary step of projecting the image of the sun directly upon a sheet of paper, divided into squares and attached to the eye-end of a great equatorial telescope. When this is directed to the sun in a darkened dome, the solar picture is formed upon the paper as in a camera obscura, and this picture can be made as large or as small as we please by varying the lenses which project it. As the sun moves along in the sky, its image moves across the paper; and as we can observe how long the whole sun (whose diameter in miles is known) takes to cross, we can find how many miles correspond to the time it is in crossing one of the squares, and so get the scale of the future drawing, and the true size in miles of the spot we are about to study. Then a piece of clock-work attached to the telescope is put in motion, and it begins to follow the sun in the sky, and the spot appears fixed on the paper. A tracing of the spot’s outline is next made, but the finer details are not to be observed by this method, which is purely preliminary, and only for the purpose of fixing the scale and the points of the compass (so to speak) on the sun’s face. The projecting apparatus is next removed and replaced by the polarizing eye-piece. Sir William Herschel used to avoid the blinding effects of the concentrated solar light by passing the rays through ink and water, but the phenomena of “polarization” have been used to better advantage in modern apparatus. This instrument, one of the first of its kind ever constructed, and in which the light is polarized with three successive reflections through the three tubes seen in the drawing (Fig.14), was made in Pittsburgh as a part of the gift of apparatus by one of its citizens to the Observatory, and has been most useful. By its aid the eye can be safely placed where the concentrated heat would otherwise melt iron. In practice I have often gazed through it at the sun’s face without intermission from four to five hours, with no more fatigue or harm to the eye than in reading a book. By its aid the observer fills in the outline already projected on the paper.