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When motion is imparted to the test object in perimetry, a source of error is introduced. The motion of the object holder may be seen while the test object itself is invisible, and an inaccurate field is thus charted.
After seeing two instances of this error in various clinics, in which an immediate check revealed the discrepancy, I devised the following test object. It consists of a glass-headed pin on the end of a holder, the pin being bent at an angle of 90 degrees, 5 mm. from the center of the bead. When the handle is rotated between the fingers on its long axis, the bead will describe a circle of 1 cm. The bead will appear to the patient to be moving back and forth over a linear distance of 1 cm. The handle will appear stationary.
Waugh DD. AN IMPROVED TEST OBJECT FOR PERIMETRY. Arch Ophthalmol. 1939;22(3):450. doi:10.1001/archopht.1939.00860090116016