Fig. 244.—Diagram of anallatic telescope.
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563.—There is an adjustment made by sliding tubes to bring the object-glass and anallatic lens within mutual focus to ensure the parallelism of the emergent rays and to adjust magnification. This is commonly effected by means of a rack and pinion, moved by a separate key kept in the instrument case, but which should not be touched after the instrument is once adjusted by the maker, except in the case of accident. It is much better made without this rack adjustment and permanently fixed by the maker, as if it has the adjustment it is likely to be tampered with and thus defeat its object. The eye-piece adjusts to distance from the object-glass in the ordinary manner of the surveying telescope—by rack and pinion.
564.—The eye-piece of the anallatic telescope is generally made of much higher power than those ordinarily employed for levels and theodolites—25 to 30 diameters is usual. Where a diaphragm is used the subtense lines are commonly placed on a slip of glass in two or three sets, so that greater magnitude of image may be taken for objects at distances of from 2 to 7 chains with the 14-feet staff, or that the staff may be read at greater distances than 14 chains. This series of lines is distinguished as 50, 100, and 200, Figs. 245, 246 and 247; so that with this as great a distance as 28 chains with a 14-feet staff may be estimated, but this is beyond the safe power of the instrument. The intermediate line, as shown [Fig. 220], is valuable in all cases for levelling. The advantage of the increased power of the eye-piece is more than neutralized by the loss of light.
Figs. 245, 246, 247.—Subtense lines ruled on glass.
Fig. 248.—Adjustable point diaphragm with stadia points.
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565.—While many civil engineers are satisfied with a single percentage pair of subtense lines the author much prefers using the point system, [arts. 237] to 239. In this case the diaphragm, as made by the author, possesses two systems of adjustment; that shown Fig. 248 at a for the single point for altitudes, and the pair of points separated by the spring ss for subtense angles. These points adjust by separate screws top and bottom with a milled-headed key f. The two verticals are fixed permanently. These points are all made of platino-iridium, which possesses the hardness and elasticity of spring steel, and is at the same time, as far as is known, perfectly non-corrosive. In case of any light dust or moisture resting upon the points, it is perfectly safe to brush them lightly with a soft camel-hair brush to clean them. Where the 200 factor is required, a mean may be taken of two observations above and below the central point. Where 50 is required, the vertical points may be adjusted to this.