The Astrophysical journal . g attainable in the case of spots, and render a different modeof measurement feasible. It will appear, however, that in its per-fected form the instrument described in this paper is capable ofgiving results no less precise than the ordinary measuring machine. The first and simplest form of the instrument, which I devisedfor the measurement of the Kenwood spectroheliograph plates byMr. Fox at the Yerkes Observatory, consists of a metallic globe,with smooth white surface, upon which the solar image is opticallyprojected. The axis of the globe being set at the inclinat
The Astrophysical journal . g attainable in the case of spots, and render a different modeof measurement feasible. It will appear, however, that in its per-fected form the instrument described in this paper is capable ofgiving results no less precise than the ordinary measuring machine. The first and simplest form of the instrument, which I devisedfor the measurement of the Kenwood spectroheliograph plates byMr. Fox at the Yerkes Observatory, consists of a metallic globe,with smooth white surface, upon which the solar image is opticallyprojected. The axis of the globe being set at the inclination requiredby the date of the photograph, and the plate properly oriented, itis only necessary to read off the heliographic latitudes of the flocculiand their differences in longitude from the suns center, by the aid ofparallels and meridians ruled, one degree apart, on the surface ofthe globe. The results obtained with this simple device were sosatisfactory that it is still regularly employed in the measurement PLATE XVII. The Heliomicromkter THE HELIOMICROMETER 295 of the Rumford spectroheliograph plates. The essential conditionto be met is that the angular diameter of the globe, as seen from theprojecting lens, shall equal the angular diameter of the sun, as seenfrom the In the second form of the instrument, as constructed in the shopsof the Solar Observatory, two 4-inch (10 cm) telescopes were mountedparallel to one another. One of these pointed at a globe, 60 feet( m) distant, beside which stood the solar photograph to bemeasured, with its center in the optical axis of the other images of globe and plate, as formed by the two telescopes,were brought together in a single eyepiece, by means of a half-silvered prism. After the photograph had been oriented and centeredon the globe, by adjustments controlled from the observers seat,the latitudes and longitudes of the flocculi were read off with respectto parallels and meridians, ruled one degree apar
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