The tangent screen and the ophthalmoscope are the two instruments essential for the accurate diagnosis of visual disturbances which cannot be attributed to refractive errors or to changes in the transparency of the refractive media of the eye. Moreover, in cases in which the ophthalmoscopic findings are essentially negative, the tangent screen is indispensable. With the ophthalmoscope it is possible to detect altered anatomic structure (pathologic change) in the fundus. With the tangent screen and the perimeter it is possible to test altered function of the visual sensory system from the rods and cones to the calcarine fissures. The function may be depressed by causes inside or outside of this system. If one considers the variable factors of size and color of test objects, the distance at which the screen is placed, the illumination, the size and position of defects, the slope of the defects, whether there is proportion or
WALTER F. DUGGAN. TANGENT SCREEN SCOTOMETRYVALUE IN DIAGNOSIS, PROGNOSIS AND EVALUATION OF THERAPY. Arch Ophthalmol. 1940;23(2):316–339. doi:10.1001/archopht.1940.00860130354005
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