1. Light from an optical slit is brought to a focus by the mirror or lens under test. A single tensioned wire of about 0.003 to 0.006 inch diameter is placed just in front of focus and arranged symmetrically on the axis of the returning cone parallel with the optical slit. 2. The images of two parallel gauge slits, illuminated with a contrasting color, are symmetrically superimposed on the speculum image by means of an unsilvered diagonal or equivalent. The gauge slits are also arranged to look parallel to the optical slit image. 3. The eye, when placed at the focus of the returning cone, sees the shadow image of the test wire and the image of the gauge slits against the illuminated speculum. 4. By moving the test wire along the axis of the returning cone, its shadow image is expanded and made to touch or intersect the parallel gauge images at the various zonal radii of the mirror under test. The motion of the wire is a direct measure of the aberration existing between those respective zonal radii. 5. The geometry of the test is described and a few sketches show the appearances of some of the common forms of aberration.
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