The publication of Einstein’s paper on the photoelectric effect in 1905 brought him to Lenard’s attention. A brief exchange of letters took place in which interpretations of spectral lines and the photoelectric effect were discussed. These letters and those from Einstein to Lenard’s assistant Jakob Laub show that Lenard and Einstein possessed a high admiration and respect for one another. Einstein called Lenard a great master and genius; Lenard wanted to appoint a personality like Einstein as a professor in Heidelberg as late as 1913. The relationship between the two physicists took a turn for the worse when Lenard began studying the theory of relativity. In 1910 he defended the concept of ether in a lecture which Einstein characterized as infantile. After 1917 he opposed the general theory of relativity. In 1918 he published his objections in a treatise On the Principle of Rela-
tivity, Ether, and Gravitation. At that time Lenard still accepted the special theory of relativity and wanted only to modify the general theory in part. Einstein answered Lenard’s objections, Lenard responded anew, and in these publications the personal side of the conflict moved more and more into the foreground. The final break came in 1920 when a certain Paul Weyland organized an antisemitic campaign against Einstein and his physics in Berlin in which Lenard’s name was used. From Lenard’s correspondence with Stark and Gehrcke it is evident that Weyland had misused Lenard’s name. Einstein, who did not know this, attacked Lenard in a newspaper article. Lenard was deeply wounded and demanded a public apology. An attempt by Sommerfeld to reconcile the two colleagues ended with a failure. In September 1920, Einstein and Lenard squared off during a heated discussion in Bad Nauheim, and from then on Lenard’s personal antisemitism played an increasingly central role in his opposition to the theory of relativity and its founder.