 Since Millikan's work formed some of the basis for modern particle physics, it is ironic that he was rather conservative in his opinions about 20th century developments in physics, as in the case of the photon theory.Another example is that his textbook, as late as the 1927 version, unambiguously states the existence of the ether, and mentions Einstein's theory of relativity only in a noncommittal note at the end of the caption under Einstein's portrait, stating as the last in a list of accomplishments that he was "author of the special theory of relativity in 1905 and of the general theory of relativity in 1914, both of which have had great success in explaining otherwise unexplained phenomena and in predicting new ones.