How to Use a Plumbing Mirror

Object 1. To test whether or not a line of sight that is autocollimated with a plumbing mirror is vertical.

Test. Arrange an alignment telescope so that it is aimed vertically downward. Place a plumbing mirror under it, level the sphere, and autocollimate with the alignment telescope. Rotate the mirror 180 degrees in azimuth. The line of sight should still be in autocollimation.


Figure 13-34 shows how the device is constructed. The basin which holds the mercury has a spherical outside diameter of 3 1/2 in., and so it can be used in a cup mount. In contact with the oil surface is an optical flat with parallel surfaces. The upper surface of the flat has a target pattern at its center. The flat is mounted so that the target center is at the center of the sphere. In use, oil is poured over the glass so that the same refraction occurs on both surfaces of the glass, as shown in the figure.


The device is observed by an alignment telescope mounted vertically (Fig. 13-35). When the line of sight is autocollimated (or autoreflected) by reflection in the mercury, it must be perpendicular to the mercury surface and therefore vertical, even though the plumbing mirror is not level.

The alignment telescope is first plumbed as described, and then the target is brought on line. In use, the plumbing mirror is first leveled approximately with a shop level, to keep the mercury clear of the glass. The mercury is then allowed to settle for 1 or 2 min.

Because of the impossibility of obtaining an optical flat that has exactly parallel surfaces, the device may produce a slight error, as shown in Fig. 13-36. Since vertical lines of sight are almost always very short, this error is negligible for the purpose intended. If a very accurate plumb point is required on long sights, however, two points can be found by observing the pool in one position and then observing it after it has been rotated 180 degrees. The average of the two alignment telescope positions will be correct.


Excerpt From:

Kissam, Philip, Optical Tooling for Precise Manufacture and Alignment, New York, NY: McGRAW-HILL BOOK COMPANY, INC., 1962