The elements of astronomy; a textbook . Fig. 148. — The Filar Fig. 149. — Construction of the Micrometer. movable wire points from one star to another in the field ofview, the position angle is then read off on the circle, A. With such a micrometer we can measure at once the distancein seconds of arc between any two stars which are near enough § 542] THE HELIOMETER. 413 to be seen in the same field of view, and can determine the position angle of the line joining them. The available range in asmall telescope may reach 30. In large ones, which with thesame eye-pieces give
The elements of astronomy; a textbook . Fig. 148. — The Filar Fig. 149. — Construction of the Micrometer. movable wire points from one star to another in the field ofview, the position angle is then read off on the circle, A. With such a micrometer we can measure at once the distancein seconds of arc between any two stars which are near enough § 542] THE HELIOMETER. 413 to be seen in the same field of view, and can determine the position angle of the line joining them. The available range in asmall telescope may reach 30. In large ones, which with thesame eye-pieces give mnch higher magnifying powers, therange is correspondingly less — from 5 to 10. When the dis-tance between the objects exceeds 1 or 2, however, the filar-micrometer becomes difficult to use and inaccurate, and wehave to resort to instruments of a different kind. 543. The Heliometer. — This instrument, as its name im-plies, was originally designed to measure the apparent diameterof the sun, and is capable of measuring with extreme precisionangular distances ranging all the
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