436.—Micrometer Reading Theodolites.—The favor with which the smaller micrometer reading theodolites have been received and the ever increasing demand for them owing to their much greater accuracy has induced the reviser to introduce a whole range of these instruments. In many cases where greater accuracy is required for horizontal than for vertical angles, the micrometers are only fitted to the horizontal circle and the vertical circle has verniers as usual. A useful instrument for general surveying without any unnecessary elaboration is shown below at Fig. 183.
Fig. 183.—Stanley's micrometer transit theodolite.
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The 6-inch instrument reads to 5 seconds of arc on the horizontal and to 10 seconds of arc on the vertical circle, and the 5-inch instrument to 10 seconds of arc on the horizontal and to 20 seconds of arc on the vertical circle.
A full micrometer reading instrument is shown at Fig. 184, the 6-inch reading to 5 seconds of arc on both circles and the 5-inch reading to 10 seconds of arc on both circles.
Fig. 184.—Stanley's full micrometer transit theodolite.
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A specially light form of micrometer reading transit (Fig. 185) has recently been designed by the reviser which has met with much favour. It has a 4½-inch horizontal circle reading by micrometers to 20 seconds of arc which may be approximately read by mental subdivision to 5 seconds, and a 4-inch vertical circle reading by verniers to single minutes. It is also made micrometer reading to 20 seconds of arc to both circles. The compass is of circular form reading by microscope to ¼-degrees, which may easily be estimated to a third of this.