334.—Micrometer Microscope, for Reading Subdivisions.—Where more exact reading is required than is possible with the vernier, as in the case of the reading of circles 10 inches or more to seconds, a micrometrical microscope is employed, which gives means of measuring the distance from line to line of the division upon the limb by the displacement of a web, point, or line moved by a fine screw with a divided head.
The great demand of late years for reducing the size and increasing the accuracy of theodolites has induced the highest class makers to introduce micrometer reading instruments of six, five, and even four-inch circles, and their accuracy is far greater than is possible with any instrument of the same size that reads by verniers. Of course the workmanship in these instruments has to be of a higher order, and the reviser estimates the accuracy of the micrometer through magnification and the necessary refined workmanship to be at least four times as great as the vernier reading, with the advantage that the micrometer is much more certain and easier to read.
335.—The construction of the reading micrometer as originally designed by Troughton has not been materially modified in those in general use. Certain refinements have been introduced for astronomical work: these are sometimes expensive and often cumbersome, so that they need not be considered in relation to surveying instruments.
336.—In all cases where micrometers are used, the structure of the framework of the instrument which carries them should be made extremely rigid, as very minute deflections or vibrations render the reading to seconds of arc impossible. The number of micrometers applied to a circle is generally 2, 3, or 5.
337.—If a circle is to be read by micrometers, the vernier is generally dispensed with. The circle is usually divided to read in 5′. The first approximate reading used to be taken by a single index line with the aid of the ordinary reader, [Fig. 130]. From the index line the degrees or minutes were taken to the last 5′ line indicated. Since the introduction of high-class engraving machinery the figuring is made at each degree and is clearly read in the microscope, so that the index reader is unnecessary. This engraving is quite a nice piece of work, as to figure from 0 to 360 means nearly a thousand figures, and on a 5-inch circle these have to be less than 1/100th of an inch high. Only the highest class makers are able to do this work. When a microscope is adjusted to one line it should be observed that all the other microscopes upon the same circle should also read exactly to a line that should be true from microscope to microscope to the arc they subtend between each other.
Fig. 135.—Side elevation of Troughton's micrometer.
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Fig. 136.—Section of micrometer.