Adjustments.
Adjustment for Height.—The erect and invert images sometimes do not touch the dividing line with similar point so that one image reaches this line before the other. In this case, lay the range finder on an object having a sharply defined horizontal line or very prominent point, and bring the images of this point exactly opposite each other by means of the measuring roller (M). The two images are then brought to the dividing line, the lower image by means of the elevation worm knob (c), and the upper image by means of the halving adjusting roller (J).
Adjustment for Range.—Three methods are available for adjusting the range finder for range; the artificial infinity method, using the adjusting bar; the actual infinity method using the sun, moon, star, or a very prominent distant point; the known range method, using a point the range to which is known. Of the three, the first is the best and should habitually be used.
By the Adjusting Bar.—Set up the range finder; take the adjusting bar and place it 100 yards from the instrument, sighting through the peep sight (y) of the adjusting bar and moving it until the range finder appears in the center of the field of view. Set the range scale at infinity by turning the measuring roller (M), then make an observation on the adjusting bar. The right hand line of the upper image of the adjusting bar and the left hand line of the lower image should now be coincident, in which case the instrument is in adjustment (Fig. 10). If not, bring these lines in coincidence by turning the key which fits the square shaft (K), which operates the range correction dial (L). This operation should be repeated at least three times, the reading of the range correction dial being noted each time. Now, set the range correction dial at the mean of the three readings. Remove the key. The instrument is now in adjustment. Before using, always note the reading of the range correction dial.
THREE STEPS IN MEASURING A DISTANCE.
By the Actual Infinity Method.—This method is in all respects the same as the one with the adjusting bar, except for the images. After the instrument is set for infinity, actual coincidence is made as in paragraph 99.
By the Known Range Method.—Operate the measuring roller (M) until the range dial reads the actual distance to the object. After the range finder has been set for this known range, actual coincidence is obtained by using the key which operates the range correction dial.