The ioskiascopy test is a sensitive subjective test for astigmatism. The apparatus consists of a revolving disk set in a fixed frame, carrying two scales, an outer one for use when testing with plus cylinders and an inner one for use when testing with minus cylinders. The disk carries two curved lines which form the two limbs of an arrow. Each limb is the arc of a parabola so cut that the tangent at one end is at right angles to the tangent at the other end.
In essence, the arrow is a fan chart, but one of extreme fineness. If one imagines the lines of a fan chart, which radiate from a common center, tacked end to end, one gets roughly a single curved line. In fact, each half of the fan chart with the lines thus arranged forms a single curved line. There is, however, an important point