 In 1958, de Vries showed that baffling anomalies in the carbon-14 dates, observed by Willard Frank Libby for Egyptological samples, were in fact systematic anomalies on a global scale, represented in the carbon-14 dates of tree rings.This phenomenon has been called the "de Vries effect".The correspondence with tree rings, which can be counted (one ring for each year), led to a recalibration of radiocarbon dating that was a large improvement in the accuracy.