D O C U M E N T 3 1 1 J U L Y 1 9 2 2 2 4 1 diately outside the hollow sphere and that would remain so if one exchanged the hollow sphere for a massive sphere with a finite spatial density. The critical value , according to this consideration, would be the one that makes the velocity of light and the inertia in the sphere’s interior finite without the influence of other masses. I originally started out with considerations of this kind, because I wanted to show that the Lorentz contraction (just like oblation in rotations) can be regarded as the effect of distant masses. I later moved to the attempt you know, because I wanted to smooth the overly abrupt transition at r = a. Now it almost seems as if the earlier consideration had greater legitimacy! May I also ask you to send the two carbon copies back to me, at your conve- nience (there is absolutely no hurry!)? The second must make its way into the wastepaper basket. In asking you again to forgive me for the disturbance and in utmost admiration and gratitude, I am yours very truly, George Jaffé. 311. To Richard Eisenmann Berlin, 27 July 1922 Highly esteemed Doctor Eisenmann, After looking at your device for sustained stimulation of piano sounds and hav- ing admired the work you achieved, I must write you briefly to give you my view on it.[1] 1) The tones generated by your device have an extraordinary artistic attraction. Your method ought to lead to a valuable enrichment of the means of musical expression. 2) With your device you overcame all the principal problems standing in the way of electromagnetic stimulation of strings in such a way that practical construction of this device should not pose serious difficulty in the hands of a skilled design engineer. 3) Your main accomplishment, which seems to me to be of considerable techni- cal importance, even disregarding the problem of sound production, is the solution to the problem of regulating the rotational speed of a rotating motor by a contact pendulum in such a way that the regularity of its motion absolutely corresponds to that of the pendulum.[2] You thus manage to have the strings stimulated at exactly 2m a ------- 1 =