Fig. 133.—Reflecting surface reader.
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332.—Shades for Vernier.—It is very general on the Continent to place the divided reading of the circle and its vernier on a plane perpendicular to the axis, [Fig. 128], and to place the reader at a fixed angle for down-the-line reading, the object-glass of the reader being constructed to focus parallel rays. In this way the division of the circle is followed into its vernier or vice versa. In this case the silver may be shaded by ground glass, which gives a soft, pleasant reading in most lights. The general arrangement is shown, Fig. 134; L limb, V vernier, S shade of ground glass, M reader. Objection is made to glass shades by civil engineers as being too delicate and liable to fracture, with risk of the particles of glass getting into the working parts of the instrument. To obviate this the author has made the shade of a piece of thin horn or transparent ivory, which appears to answer very well and to save this risk.
Fig. 134.—Oblique reading microscope with shade, French plan.
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333.—For ordinary instruments with no provision for shading, a piece of transparent horn about 2¼ inches by 1¼ inches may be carried in the waistcoat pocket, and will be found a great comfort if held over the vernier when the lines appear glary, or the horn may be placed in a pocket frame with the case containing reflector for bubble reading, [Fig. 52]. In large theodolites, used for geodetic surveys, the object-glass of the micrometer microscope is sometimes surrounded by a thin belt of turned ivory. This throws a very soft light upon the divisions.