In practicing perimetry one frequently needs some rapid, simple and yet accurate method of determining the size and location in the field of vision of a scotoma or blind spot when the usual tangent screen laid off to scale in polar coordinates is either not available or inadequate. For example, in emergency cases when examination must be made away from office equipment or in hospital cases in which it is inexpedient to move the patient a blackboard or black shade or even a piece of black paper pinned against the wall is often available, and its use is preferable to that of the small portable hand perimeter, if some means were at hand of quickly locating or measuring the size of the scotoma found. More frequently, it is desirable to vary the distance of the screen from the eye, either for comparative purposes in a given case or to meet
Wentworth HA. THE TANGENTOMETER: A New, Portable Instrument for Charting Tangent Screens at Various Distances. Arch Ophthalmol. 1934;12(3):403–406. doi:10.1001/archopht.1934.00830160107012
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