2 6 2 D O C . 3 1 6 F E B R U A R Y 1 9 2 0 only one solution: the nature of this course as a private lecture series be retained students who have accredited themselves receive reserved seats the remaining au- ditors gain admission as space allows. Although this solution does not follow the regulations, I do not see any other possibility that satisfies all legitimate wishes. In case that this is not acceptable, I will discontinue the course. In this latter case, I ask you kindly to issue directions in the sense that the course fees be reimbursed. Requesting prompt notification about the decision reached, I send you kind re- gards, yours sincerely. P.S. In the event of approval, I would place a notice in the newspaper.[2] 316. From Edouard Guillaume Bern, 15 February 1920 Dear Einstein, I am very glad to find that you have kept your good humor despite the difficult times.[1] But you shouldn’t make it so easy for the rascal to reply![2] “A concept,” says Einstein with full justification, “exists for a physicist only when there is a possibility of finding out in a concrete case whether or not the con- cept applies.”[3] I am therefore going to ask you to tell me what you mean by a “clock (?) judged (?)” from K.[4] 1° The statement has no physical meaning then it is of no interest to a physicist and you are not allowed to draw any conclusions from it about spectrum lines. 2° The statement has physical meaning but then you must specify exactly what kind of experimental arrangement is needed in order to see anything. One more remark: that a clock’s carrier is at rest with reference to does not say that no motion is occurring within the interior and surroundings with reference to . For otherwise, how would the clock function? No motion: no clock. The train of light waves moving along a plane, which you mention in the second exercise, surely originates from a light source. When you introduce it explicitly, the apparent discrepancy between the two cases disappears, in that you are likewise in- troducing a light source as a “clock” into the first case. Are you still not convinced? Fine! Examine closely the following systems of equations:[5] K′ K′