differently so that one set of transparent secondaries is lying in the plane and another set of transparent secondaries, almost invisible to the observer and obscured by a third set of opaque secondaries, slants down into the plane. The third set is opaque and slants up toward the viewer away from the plane. Unetched, the stereoscopic field of these tiny structures was confusing and only two branches were recognized; partially etched, the three branches were distinguishable by reason of contrast differences. 16, 695 (1950).

No abstract