This lens is shown at A with the halves widely separated by the double threaded micrometer screw B, which carries them apart symmetrically. The ocular proper is shown at C.
Double image micrometers are now mainly of historical interest, and the principle survives chiefly in the heliometer, a telescope with the objective divided, and provided with sliding mechanism of the highest refinement. The special function of the heliometer is the direct micrometric measurement of stellar distances too great to be within the practicable range of a filar micrometer—distances for example up to 1½° or even more.
The observations with the heliometer are somewhat laborious and demand rather intricate corrections, but are capable of great precision. (See Sir David Gill’s article “Heliometer” in the Enc. Brit. 11th Ed.). At the present day celestial photography, with micrometric measurement of the resulting plates, has gone far in rendering needless visual measurements of distances above a very few minutes of arc, so that it is somewhat doubtful whether a large heliometer would again be constructed.
Fig. 132.—Filar Micrometer. (Courtesy of J. B. Lippincott Co.)
The astronomer’s real arm of precision is the filar micrometer. This is shown in outline in Fig. 132, the ocular and the plate that carries it being removed so as to display the working parts. It consists of a main frame aa, carrying a slide bb, which is moved by the screws and milled head B. The slide bb carries the vertical spider line mm, and usually one or more horizontal spider lines, line mm is the so-called fixed thread of the micrometer, movable only as a convenience to avoid shifting the telescope.
On bb moves the micrometer slide cc, carrying the movable spider line nn and the comb which records, with mm as reference line, the whole revolutions of the micrometer screw C. The ocular sometimes has a sliding motion of its own on cc, to get it positioned to the best advantage. In use one star is set upon mm by the screw B and then C is turned until nn bisects the other star.
Then the exact turns and fraction of a turn can be read off on the comb and divided head of C, and reduced to angular measure by the known constant of the micrometer, usually determined by the time of passage of a nearly equatorial star along the horizontal thread when mm, nn, are at a definite setting
apart. (Then r = (15(t′-t) cos d)/N where r is the value of a revolution in seconds of arc, N the revolutions apart of mm, nn, and t and d as heretofore.)