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In testing the field of vision it is desirable to have the same illumination of the test object in all parts of the field, and to be able to diminish the intensity of this illumination by a known and measured amount when we wish to test the field for white or colors in cases of beginning atrophy or other conditions where there is a marked difference in the extent or shape of the field of vision when tested with a full and with a diminished amount of light. This seems to be best accomplished by making the tests in a dark room and using two incandescent electric lights of one candle power each for the test object and the fixation point. A Förster's perimeter with a broad semicircular arc of hard rubber is arranged so that one lamp is placed in the center of the arc and about an inch
WILLIAMS CH. A NEW PERIMETER.. JAMA. 1898;XXXI(14):767. doi:10.1001/jama.1898.92450140021001i